Written as a fetishy sequel of sorts to the 'good end' of Splorch Bucket's Better Living Through Thaumaturgy (https://pastebin.com/z2TdsYQ5).
Reading it is not required, but I do recommend it for any fans of cake-expandy goodness.
 
>Princess Celestia let out a contented sigh as she closed and locked the door to her private bedchamber.
>Another long day of greeting visitors, addressing subjects and moderating disputes was over.
>Were it a normal day, the princess would begin making some evening tea before going to bed.
>Tonight, however, she had a much more delicious treat in mind.
>All thanks to her faithful student, (Princess) Twilight Sparkle.
 
>Celestia trotted over to her bed, climbing atop the red velveteen covers.
>Once comfortable, she reached out with her magic, grasping the long wooden box hidden under the bed.
>The box rose up before her, and she gently set it down on the sheets beside her.
>Carefully, the princess creaked open the hinged lid, revealing the dozens of small pastries and cake slices in their felt-lined compartments.
>Minus two, of course.
>Celestia smiled as she sensed a faint freshness spell around the box to keep the treats from going stale.
>Her student was as forward-thinking as she was generous, and it made the princess feel a twinge of warm pride.
 
>Each dessert was carefully labled in Twilight's neat hoofwriting.
>Celestia licked her lips and shifted around on the bed as she surveyed the sweets, looking for one which most struck her fancy.
>Lying on her side, the princess stroked a hoof over her shapely flanks and soft, flat middle.
>It had taken a lot of magic, and a few laps around the castle grounds, to work off all the pudge she had put on from her birthday slice of hazelnut coffee cake.
>Kibbutz had nearly fainted when he had found her swollen form after the celebration.
>He had sternly made her promise not to eat another bite from Twilight's present.
>Royal duties required the keeping of a royal figure, after all!
>Celestia made a patronizing smirk as she remembered her assistant's warnings.
>It's a good thing he hadn't been alive to see her eating habits 200 years ago.
>The sun princess was well entitled to indulge herself every now and again, thank you very much.
>Not to mention this was a gift from her prize pupil, now the Princess of Friendship!
>It would almost be a crime to let it go to waste...
>And though Celestia would never admit it, there was a certain thrill involved in defying her courtiers' wishes, even though she thoroughly outranked them.
 
>After a few moments of reflection, Princess Celestia levitated a square slice of cake from the box.
>The small attached tag read "Chocolate Fudge - 205"
>The slice appeared to be chocolate cake with thick layers of dark fudge, covered by a ganache of smooth chocolate icing.
>Rolling onto her back, the princess licked her lips, her mouth watering in anticipation.
>Though vanilla was publicly her favorite flavor, Celestia always harbored a sinfully strong desire for dark chocolate.
>She levitated the slice closer, catching a whiff of fudge with her nose.
>With a levitated fork that she had carefully lifted earlier from the castle kitchens, she carved a healthy chunk off the top of the cake.
>The princess banished a sudden twinge of guilt for breaking her promise as she opened her mouth and forked the cake inside.
 
>As it touched her tongue Celestia was overwhelmed with the taste of rich chocolate, richer than any she had tasted in decades.
>"Mmmph!" she cried through pursed lips as fudgy flavor melted over her taste buds.
>Her leg twitched as she felt a twinge of warm arousal.
>Celestia began chewing as the fudgy cake began warming and expanding in her mouth.
>Her head was swimming as she took in the salty, heavy sweetness of Twilight's dessert.
>The alicorn intensified her chewing as chocolate fudge covered her tongue and began pressing out her cheeks.
>Celestia pushed the cake to the back of her palate, her neck muscles forcing the bite down her throat.
>In the back of her mind, the princess wished that the cakes expanded slower, so that she would have more time to savor their incredible flavor.
>Her neck bulged as the expanding food traveled down her gullet.
>Finally Celestia felt the cake pass through and settle heavily in her stomach.
>The princess took a few deep breaths before recovering control of the fork.
>She quickly plunged the utensil back into the remaining slice, forking off an even bigger bite of fudge and heaving it into her mouth.
 
>Princess Celestia began desperately chewing the new bite of fudge cake, her eyes screwed tight as she basked in its blissful flavor.
>The room filled with the sound of smacking lips and food-muffled moans as the princess continued her private indulgence.
>The white alicorn's cheek muscles burned as she forced herself to chew the rich, dense dessert.
>Her hoof absentmindedly stroked her warm, rumbling middle.
>Celestia forced the second bite down, then immediately plunged her fork in for another.
>Tears appeared at her eyes as a third explosion of flavor hit her palate.
>Once the third bite was gone, the princess levitated the small plate closer, opened wide, and dumped the remainder of the slice inside.
>The alicorn struggled to chew, her jaw aching with soreness
>Rich fudgey residue coated the inside of her mouth.
>The last bite of cake continued to expand in her struggling mouth, her cheeks bulging full with fudge.
>The cake filled her mouth, and began pushing her lips open to dribble crumbs out her mouth.
>Celestia finally unleashed her magic to push the growing foodstuff back, magically stretching her throat to force it down her throat and into her stomach.
>The princess let out a breath of relief as the last bite settled heavily in her belly.
>Her tongue began licking crumbs and fudge residue from the roof of her mouth, gulping them down before they expanded themselves.
>The alicorn cursed herself for not bringing a pitcher of milk with her to bed.
>Her mouth mostly clean, Celestia let out a contented sigh and focused her attention towards her middle.
 
>The princess massaged the small bulge in her middle as the fudgy cake expanded inside her stomach.
>She blushed, letting out a throaty BURP as emptiness was pushed out of her, replaced with a growing fullness.
>Celestia's breath quickened as she felt her stomach stretching to accomodate the cake.
>Her hooves roamed over her belly as it slowly swelled around her filling stomach.
>The princess felt her nethers warm as her middle grew larger, heavier, denser, fuller.
>She felt full, then fuller, then stuffed, then overstuffed, as her insides were packed with more and more cake.
>Celestia lay back with her eyes unfocused, her mind overwhelmed with the sensation of tremendous overfullness.
>Her breathing grew ragged as her stomach pushed against her lungs.
>The princess' body sank down into her mattress, weighed down by hundreds of pounds of dense, packed cake.
>Her belly was almost perfectly spherical, and bulged out pregnantly in every direction, pinning the monarch helplessly to the bed.
 
>Finally, mercifully, the cake stopped expanding.
>The burstingly full princess lay in a stupor, letting out low moans with each gasping breath.
>Celestia slowly recovered her wits, her hooves daring to touch the drum taut flesh of her cake-engorged gut.
>She was so full, so taut, that she dare not attempt to pull herself out of bed.
>As her senses returned, Celestia felt the the sheets at her thighs were damp with more than sweat.
>Burning arousal seeped back into her brain as the princess cradled her swollen middle.
>Willing herself to concentrate, the princess focused her magic and reached out to pull a long, dark object from within her bedside table.
>Celestia levitated it over her swollen form, pushing it tenderly between her splayed legs.
>Sparks flashed into her mind as she thrust it in, and felt the motion press against her super-packed stomach.
>The princess cried out in bliss, retracting the dildo before eagerly thrusting it again.
>As Celestia began to pleasure herself, she tasted the flavor of chocolate fudge on her lips.
>She was going to have to write Twilight a VERY gracious thank-you note after this.
 
Possibly to be continued...she's still got a whole box of cakes left~