Chocolate Kiss Thumbprint Cookies With Walnuts

Ingredients

• 1/2 cup butter, softened
• 2/3 cup sugar
• 1 large egg, separated, room temperature
• 2 tablespoons 2% milk
• 1 teaspoon vanilla extract
• 1 cup all-purpose flour
• 1/3 cup baking cocoa
• 1/4 teaspoon salt
• 1 cup finely chopped walnuts
• FILLING:
• 1/2 cup confectioners' sugar
• 1 tablespoon butter, softened
• 2 teaspoons 2% milk
• 1/4 teaspoon vanilla extract
• 24 milk chocolate kisses