• Preheat the oven to 350 degrees. 
• Grease the bottom and sides of an iron skillet and set aside.
• In a small bowl whisk together the dry ingredients – flour, baking powder, baking soda and cornstarch. Set aside.
• Using a mixer beat the butter until creamy then add in the sugar and beat until light and fluffy. Usually about 2 minutes.
• Add the egg, egg yolk and vanilla to the sugar and butter mixture and combine fully.
• Next add in half of the flour mixture and stir then add in the remaining half of the flour mixture and continue stirring until incorporated. Fold in the sprinkles.
• Spread the cookie dough into the prepared skillet and press it into an even layer and smooth the top with a rubber spatula.
• Bake for 25-30 minutes until lightly golden on top.
• Once done remove from oven and allow to cool for 15 minutes as you prepare the frosting.

How To Make Buttercream Frosting

• Add the butter, powdered sugar and vanilla in a medium-sized bowl and beat for one minute until smooth. 
• Then add one Tablespoon milk and beat for 30 seconds. Adjust thickness as needed by adding additional sugar or milk.
• Scoop the frosting into a piping bag fit with a star tip.

Add Frosting To Cookie

• Using the star piping tip, pipe the frosting around the edge of the cookie. Make short 2″ swirls. Then top with sprinkles..