Bake the cake and prepare the fillings and frostings according to the recipe instructions.
Using your hands, crumble the baked cake into small pieces. It should be around 2 ½ cups of crumbled cake. (You can make this cake from a 1x batch of my cake recipes.
Assemble the Cake Cups
Layer the cake, fruit filling (if using), and buttercream in each cup. There should be enough room for 3 layers of each (cake, fruit filling, and whipped cream x 3 in each jar).
As you fill them, keep an eye on the sides of the cups, piping or spooning against the sides as needed for the prettiest-looking layers. To avoid too much buttercream, only pipe the buttercream right against the inside of the cup's edges.
Stick a disposable spoon into the cake cup, and close the cup by placing the end of the spoon through the straw hole in the cup's lid.
Keep refrigerated until 1-3 hours before serving, then allow them to come back to room temperature. Enjoy!