Preheat oven to 350ºF. Spray a 8 X 8 baking pan with cooking spray and set aside.
In a large bowl cream together the butter and sugar together until very pale, light, and fluffy, about 3 minutes. Beat in egg, yolk, and vanilla until well mixed.
To the bowl add salt, baking powder, baking soda, and cornstarch. Mix well. Gradually add the flour on low speed until just combined. The dough will be very thick. Stir in sprinkles.
Press into the prepared baking pan and bake for 25 minutes (about 5 - 10 minutes longer if using gluten free flour) until a toothpick inserted in middle comes clean. The bars will be lightly browned. Allow the bars to cool completely on wire rack before frosting. They will sink slightly in the middle as they cool.
Tip: This makes a very thick layer of frosting (which I have no problem with!), but if you'd prefer a thinner layer, half the recipe.
In a large bowl beat the butter until light and creamy. Add the powder sugar, heavy cream, vanilla, and pinch of salt gradually on low speed until mixed in. Increase mixer to high and beat until very light and fluffy, about 7 minutes. Tip: Beating for a long time is what makes this frosting extremely fluffy. Mix in food coloring, if using.
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Tips for Making The Best Sugar Cookie Bars
For a slightly different flavor substitute almond extract for the vanilla.