Chocolate Frosted Sugar Cookie Bars

INGREDIENTS

For the sugar cookie bars::

1 3/4 cups

all-purpose flour

1 1/2 tsp

cornstarch

3/4 tsp

baking powder

1/2 tsp

baking soda

1/4 tsp

fine salt

1 stick

unsalted butter, at room temperature

2 oz

cream cheese, at room temperature

3/4 cup

granulated sugar

large egg plus 1 egg yolk

1 tsp

vanilla extract

For the chocolate frosting::

1 1/2 sticks

unsalted butter, at room temperature

1 1/4 cups

powdered sugar, sifted

1/3 cup

cocoa powder, sifted

3 tbsp

heavy cream

1 tsp

vanilla extract

1/8 tsp

salt

Rainbow sprinkles