Preheat the oven to 350°F. Line large baking sheets with parchment paper or silicone baking mats.

In the bowl of an electric mixer, beat the butter, sugar, and vanilla on medium high speed until light and fluffy, about 2 to 3 minutes. Add the flour and salt and beat until just combined.

Using a spring-loaded cookie scoop, shape the dough into 1-tablespoon balls. Dip each ball in beaten egg whites, then roll in the chopped walnuts. Press the nuts to adhere to the dough. Place on prepared baking sheets, spacing about 2-inches apart.

Bake for 12 minutes, until cookies are puffy.

Remove from the oven and make an indentation in the centers with the blunt round end of a wooden spatula or spoon. Continue to bake for about 2 to 3 minutes, or until the edges are golden brown.

Remove from oven and spoon about 1/2 teaspoon of preserves into each indentation. Let cool on baking sheets for 5 minutes before removing the cookies to wire racks to cool completely before serving.