clues
to make the cake:
preheat the oven to 350ºf. grease and line the bottoms of four 8-inch cake pans and a mini pony pan (if you don't have a pony pan, you can leave it out).
in a medium bowl, whisk together the flour, cornstarch, salt, and baking powder.
in a stand mixer fitted with the paddle attachment, cream together the butter and sugar until light and fluffy, 3 to 4 minutes. add the egg whites, one at a time, mixing well after each addition. add the oil and the vanilla.
with the mixer running on low speed, add the dry mixture and the milk in two or three alternating batches and mix until just barely combined, using a rubber spatula, gently fold in the sprinkles until they're evenly distributed. distribute the batter between the pans.
bake until a toothpick inserted into the center comes out clean. begin checking the pony for doneness at 15 minutes and the cakes at 25 minutes.
let cool in the pans for 10 minutes and then turn onto a wire rack to cool completely.
to make the frosting:
in a standing mixer fitted with a paddle attachment, beat the butter until smooth and then gradually beat in the powdered sugar. add the salt, vanilla, and milk and beat to combine.
to assemble:
once the cakes are cooled, use a biscuit cutter to cut out 3" circles from the centers of two of the layers. use these circles to make a mini cake! or just eat them. place one of the uncut layers on a plate or cake board and spread on a layer of frosting, then stack up the two cut layers on top of that with a layer of frosting in between. fill the hole to the top with m&m's, frost the top of the cake and then top it with the remaining uncut cake layer. frost it all over.
decorate your pony as desired: decorate one side of him while he's laying flat on a pan, refrigerate so the frosting firms up, and then stick wooden skewers in the legs. use these to attach him to the cake, and then once he's upright, decorate the other side.
add flowers, succulents, sprinkles, dollops, or any other decorations that you'd like and serve!