Optional: sprinkles, colored sugar, crushed walnuts (if using walnuts, roll dough in egg whites before rolling in walnuts. If just using sugar, no need for egg whites)
2 cup butter
6 cups sifted powdered sugar
2 teaspoons vanilla
3-4 tablespoons milk
Food coloring (optional)
Cream butter, sugar, and egg yolks. Add the vanilla.
Gradually add flour until combined.
Using a 1-inch cookie scoop, roll dough into balls and place onto a silicone baking mat or parchment paper (I prefer the silicone mats).
Add thumb print to each cookie, then roll edges in choice of decoration (sprinkles, sugar, etc)
Bake at 350 for 14-15 minutes.
Cream butter until white and fluffy.
Add powdered sugar and vanilla (also add the food coloring now if you are using it).
Gradually add in milk until desired consistency.
Pipe icing on to cookies. I used a piping bag to pipe the icing, but you could also use a Ziploc bag and snip the corner. I prefer the piping bags so that I can use different tips for different designs.