3/4 cup unsalted butter, room temperature
3/4 cup dark brown sugar
1/4 cup sugar
1 egg
2 tsp vanilla extract
2 cups all purpose flour
2 tsp cornstarch
1 tsp baking soda
1/2 tsp salt
1 1/4 cups semi-sweet chocolate chips OR about 1 cup of chocolate chunks

Vanilla Buttercream:

1/2 cup butter
1/2 cup shortening
4 cups powdered sugar
1 tsp vanilla extract
2-3 tbsp water