Preheat oven to 350 degrees. In a large mixing bowl, whip together butter, cream cheese, and sugar for 4 minutes or until light and fluffy.
Add egg, egg yolk, and vanilla and whip for 1 minute longer.
Stir in flour, baking powder, and salt. Mix just until combined. Press sugar cookie dough into a 9 x 13 baking pan (I use an aluminum pan). You may use parchment paper on the bottom of the pan for easy removal.
Bake for 24-28 minutes. Watch carefully because you want them to be slightly underdone as they will firm up as they cool. Set aside to cool.
Cream Cheese Frosting:
In a mixing bowl, cream together butter and cream cheese for 2-3 minutes. Add powdered sugar, cream, and vanilla extract and whip for 2-3 more minutes. Fold in food coloring, if using.
Spread over cooled sugar cookie bars. Cut into squares and serve.