Preheat oven to 350 F. Line baking sheets with parchment paper or a non-stick baking mat. In a large bowl, beat together butter and sugar until light and fluffy. Beat in egg yolks, vanilla extract and salt. Gradually stir in flour.

Form dough into 1-inch diameter balls. Dip in lightly beaten egg whites, then roll in nuts. Place 1 inch apart on prepared cookie sheets. Press down center of each with thumb.

Bake for 16 to 18 minutes, or until golden brown. Cool on baking sheet for 5 minutes, then remove to a wire rack to cool completely. Just before serving, fill centers of cookies with strawberry preserves.