Preheat oven to 350℉ lightly grease a baking sheet.
Using a hand mixer, beat butter until smooth. Add sugar and mix until fully incorporated. Add the egg yolk and vanilla. Set bowl to the side.
In a separate bowl, whisk together flour and salt. Combine the flour mixture with the wet ingredients and mix until combined.
Roll the dough into 1-inch balls. If the batter is too soft to roll, refrigerate for 20-30 minutes.
In a small bowl, whisk egg white until frothy. Dip each ball into the egg whites and roll into finely chopped nuts.
Place dough balls on the prepared sheet 2 inches apart. Use the tip of your thumb to indent a small hole in the center and fill with 1/2 teaspoon of jam. If using hazel nutter spread, fill the thumbprints after baking.
Bake for 13-15 minutes. Remove from pan to cool. Store in an airtight container.