Preheat the oven to 350F. Add a box of cake mix to a large bowl.
Add canola oil, eggs, and water to the cake mix. Mix until fully combined.
Spread out the batter between two greased or non-stick rectangular cake pans. It will be a relatively thin layer.
Stick both cake pans in the over and watch them closely - you'll only want to bake for about 20 minutes since the layers are thinner. Check to see when they are done with a toothpick.
Pull both cakes out of the oven and let completely cool.
Place parchment paper or a non-stick mat on the counter and flip over both cakes onto the counter. If you need help releasing the cakes, use a spatula to slide under and assist you. Try to avoid breaking the cake, but if you do a little bit, it's fine.
Use the cookie cutter to cut out as many circles as possible. Using the size circle cutter I had, which was about 1 1/2", one box made 20 cakes (so I was able to cut out 40 circles).
Add a layer of frosting using your Pillsbury Filled Pastry bags. Make a circle on one cake round.
Place another cake round on top and then add five "squirts" of frosting on top. Just place the pastry bag down on the cake, press frosting out and pull up.
Repeat until all of your mini cakes are done. They are ready to serve!
Video
Notes
You can keep them in a fridge for a few days if you need to make them beforehand.