• Preheat oven to 350 degrees. Spray a 10" tart pan with non-stick baking spray, and set aside.
• With a hand or stand mixer, cream together the butter and sugar until creamy and fluffy. Add the egg and yolk, and vanilla. Mix until combined.
• In a separate bowl, whisk together the flower, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, stopping the mixer to scrape down the sides of the bowl. Dough will be thick and slightly stiff. Fold in the rainbow sprinkles, semi-sweet chocolate chips and white chocolate chips. Mix until just combined.
• Evenly press the dough into the prepared tart pan. Bake at 350 for 26-30 minutes until light golden brown. Remove from oven and let cool to room temperature in the pan. Once cool, you can remove the outer ring and under portion of tart pan. Make sure cookie is completely cool before frosting and decorating.

Frosting:

• With a hand or stand mixer, cream together the butter and vanilla. Gradually add in the powered sugar, one cup at a time, stopping to scrape down the sides of the bowl. Gradually add in milk 1 Tbsp at a time. Add in the food coloring of your choosing. For a deeper color, add more coloring. Frosting should be a thick consistency. Pipe on the frosting (I used a Wilton 1M tip). Decorate the frosting and cookie with additional rainbow sprinkles.