Preheat oven to 350 degrees F. Spray a mini muffin pan with cooking spray to prevent sticking and set aside.
In a stand mixer, combine butter and cream cheese on medium speed until light and fluffy; 1-2 minutes. Add egg and vanilla and blend until well combined.
Stir in half of the cake mix until blended, then stir in the remaining cake mix until cake mix is completely blended.
Scoop one tablespoon of batter into each mini muffin cup (using a cookie scoop works best) and press into the cups. It will fill the cups nearly to the top. Add sprinkles or M & Ms if desired. Bake for 12-14 minutes or until a toothpick inserted into the center brownie comes out clean.
If adding Hershey kisses, top the brownies with the kisses and put back into the oven for 1 minute.
Cool in the muffin pans or on a wire rack.