Preheat oven to 350°F (175°C) and grease a cookie sheet.

In a medium bowl, cream butter and brown sugar until light and fluffy. Stir in egg yolk, then stir in flour until combined.

Roll dough into 1-inch balls, dip in egg white, and place 1 inch apart on the cookie sheet. Make an indentation in each cookie with your finger or the end of a wooden spoon.

Bake in the preheated oven for 8 minutes, remove from the oven, and fill indentations with Cream Cheese Frosting. Return to the oven for another 3 to 5 minutes, or until golden brown.