Here's a recipe for thumbprint cookies that uses two egg yolks. I make them often with a variety of jellies.
1 cup butter, softened
1/2 cup granulated sugar
2 egg yolks
2 teaspoons vanilla or almond extract
2 cups flour
1/2 teaspoon salt
1/2 cup finely ground pecans or walnuts
6-8 ounces jelly (your choice)
Cream butter and sugar unil light and fulffy. Add egg yolks and extract and continue beating until well mixed. Mix flour and salt together and add gradually, mixing until well blended. Refrigerate dough for at least one hour.
Preheat oven to 350ºF and line cookie sheets with parchment paper. Using about 1/4 of the dough at a time, pinch off small pieces and roll in balls about 1" in diameter. Roll in finely ground nuts and place on prepared cookie sheets about 2" apart. Use your thumb to make an indentation in the middle of each cookie. Bake 10 minutes and remove from oven. While cookies are hot, fill each "hole" with your favorite jelly. (You may need to press the back of a small spoon in the center of the cookie to make the indentation a little deeper before adding the jelly.) Bake 5 more minutes and remove from oven. Cool for a few minutes before removing to a rack to cool completely. Store in a tightly covered container. Makes about 48 cookies.