Step 1

Preheat oven to 350 degrees F. Lightly grease a 9 inch cookie pan or cake pan with nonstick cooking spray. Place a circle of parchment paper in the bottom of the greased pan and lightly spray with the nonstick cooking spray.

Step 2

In a mixing bowl, add the butter, granulated sugar, and light brown sugar. Using a hand mixer, beat on medium speed for 3 to 4 minutes until light and fluffy.

Step 3

Add in the eggs and vanilla. Beat just until the eggs are incorporated.

Step 4

Add in the flour, baking powder, and salt. Mix on low until most of the flour is incorporated and then turn the mixer to medium. Beat just until the dry ingredients are incorporated.

Step 5

Add 1 cup of rainbow sprinkles into the sugar cookie dough and work into the batter with a spatula. The batter will be thick.

Step 6

Place the cookie dough in the center of the prepared pan and spread out using an offset spatula leaving about ½ inch to the edges to leave room for the dough to spread.

Step 7

Place the cookie cake in the preheated oven and bake for 18 to 20 minutes until the edges are lightly golden.

Step 8

Remove the cake from the oven and allow the cookie cake to continue cooling for an additional 10 to 15 minutes.

Step 9

Turn the cookie cake out onto a cake stand to cool completely. If you want the flip the cake, let it cool completely and flip it onto a plate or cake board and then slide it back over to the cake stand.

Step 10

In a large bowl, add the butter and shortening. Using a hand mixer, beat on medium speed until smooth.

Step 11

Add in the powdered sugar and vanilla. Beat on low speed until most the powdered sugar is worked into the frosting.

Step 12

Add in the creamer or milk. Mix on medium speed until the remaining powdered sugar and creamer is incorporated. Continue to mix an additional 1 to 2 minutes until light and fluffy.

Step 13

Fill the piping bag fitted with a tip 1M or 2D piping tip with the vanilla frosting.

Step 14

Pipe a border around the cake with the frosting and top with the additional rainbow sprinkles.