• Preheat the oven to 350°F. Spray a 9-inch cookie (or cake) pan lightly with non-stick baking spray. Place a parchment circle in the bottom and spray the top with non-stick spray. If using a cake pan be sure to spray the sides of the pan, and set aside.
• In a mixing bowl, cream butter and both sugars for 3-4 minutes. Add eggs and vanilla and mix until incorporated.
• In a separate bowl, whisk together flour, baking powder, and salt. Add dry ingredients to the wet ingredients.
• Fold 1 cup rainbow sprinkles into the cookie batter. The batter will be thick.
• Place cookie dough batter in the center of the prepared pan. Spread out with a spatula (an offset spatula works great). Using clean hands, press dough in the pan into a smooth layer leaving room around the edges so it has space to spread.
• Bake in a preheated oven for 35 minutes until it turns lightly golden brown.
• Remove the cookie cake from the oven and allow it to cool for 10 - 15 minutes.
• Remove the baked cookie from the pan and place onto a cake board or turntable or cake stand.

Make the Buttercream

• With a hand or stand mixer, cream butter and shortening together.
• Add the powdered sugar and vanilla and mix well.
• Add milk and whip the frosting at HIGH speed for 4-5 mins.

Decorate the Cookie

• Once cookie has fully cooled (after about 20-30 mins) fill a piping bag fitted with a tip 1M or 2D with vanilla buttercream.
• Pipe frosting around the cake to create a border and top with additional sprinkles.