Start by preheating your oven to 350 degrees fahrenhiet, grabbing all of your needed ingredients and an 8x8 baking dish.
Line your baking dish with parchment paper.
Start by combining butter, eggs, vanilla and icing sugar in an electric mixing bowl on medium speed for a few minutes.
Combine flour, cornstarch, baking powder and salt in a separate bowl. Once the wet mixture is well combined, gradually add in the dry mixture to the electric mixer. Mix well but don't over mix!
Transfer this mixture to your lined baking dish and stick in to your preheated oven for 25 minutes.
While this is baking, make the buttercream! Combine all ingredients in electric mixer using the whisk attachment and combine on medium speed until smooth.
Remove your cookie bars from the oven and let them cool completely before icing them and slicing them. Add your sprinkles at the end.
Makes about 16 bars!
