• In a large mixing bowl, beat the butter and sugar until light and fluffy, about 5 minutes.
• To the butter, add the egg yolks and vanilla, mixing until well blended. 
• Add the flour about ½ cup at a time and mix after each addition, scraping down the sides as needed.
• Transfer the cookie dough from the bowl to plastic wrap. Roll in plastic wrap and seal the ends tightly. Place the dough in the refrigerator and chill for at least 1 hour.
• When ready to bake, preheat the oven to 350 degrees. Prepare a cookie sheet with parchment paper.
• Using a small cookie scoop, portion the dough into 1 inch balls. 
• Dip the balls in egg whites and roll in your desired coating. 
• Place the cookie balls 1 inch apart on the cookie sheet. 
• Using the rounded side of a greased teaspoon make an indentation in the center of each cookie ball.
• Bake for 10 to 12 minutes. If necessary, use the rounded side of the teaspoon to reshape the cookie’s hole. Let cookies cool on the cookie sheet for 2 minutes before removing to a wire rack for cooling.
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To make ahead:

• Bake as directed but do not fill the cookies.
• Layer the cookies between layers of wax paper in an airtight container. Store at room temperature for up to 3 days or in a freezer up to 3 months. 
• Thaw the cookies completely before filling with your favorite filling.
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• Linzer Thumbprints
• Bake the cookies and allow to cool completely.
• Place 1 cup powdered sugar in a large storage bag.  
• Add the cookies, a few at a time, shaking gently to coat.
• Carefully remove the cookies from the bag and fill each with ½ teaspoon raspberry preserves.  Top with toasted sliced almonds.
• Peanut Butter
• Finely chop 1 cup peanuts.
• Roll each cookie ball in the egg whites and then in the finely chopped peanuts. Remember to make the indentation in each cookie prior to baking.
• Bake for 10 to 12 minutes and allow to cool completely.  
• Fill each indentation with ½ teaspoon creamy peanut butter.
• Chocolate Sprinkles
• Roll each cookie ball in the egg whites and then in the chocolate sprinkles. Remember to make the indentation in each cookie prior to baking.
• Bake for 10 to 12 minutes, remove from the oven, and immediately top each cookie with a chocolate kiss. 
• Christmas Stars
• Roll each cookie ball in the egg whites and then in the Christmas sprinkles. Remember to make the indentation in each cookie prior to baking.
• Bake for 10 to 12 minutes and allow to cool completely.  
• Fill a decorating bag fitted with a star tip with canned or homemade vanilla frosting. Pipe the frosting into each indentation making a star.
• Walnuts and Jam
• Finely chop 1 cup walnuts.
• Roll each cookie ball in the egg whites and then in the finely chopped walnuts. Remember to make the indentation in each cookie prior to baking.
• Bake for 10 to 12 minutes and allow to cool completely.  
• Fill each indentation with ½ teaspoon of your favorite jam or preserves.