Combine butter, oil, sugars and vanilla until smooth. Blend in eggs.
Add flour, salt, cream of tartar and baking soda until blended in.
Add sprinkles.
Press the dough into a greased pan. Optional: sprinkle sprinkles over the top of cake and gently press onto cake with parchment paper.
Bake 20 minutes*.
Notes
Baking time will vary depending on the pan you use. Reduce baking time for thinner cookie cakes checking in at 15 minutes and increase for thicker cakes if you want them more done. When adding dough into the pan, careful to not overfill and factor in 1/3 of dish for the cake to riseTry my super easy buttercream recipe if you’d like to frost your cake.Forgot to take out your butter? To soften butter directly from the refrigerator, place the butter unwrapped in the microwave for 10-15 seconds.