Preheat oven to 350°F. Line baking sheets with parchment paper.
In the work bowl of a food processor, combine walnuts, pecans, ½ teaspoon (1.5 grams) salt, and ½ teaspoon (1 gram) nutmeg. Pulse until combined and nuts are finely ground, about 30 seconds. Transfer mixture to a small bowl, and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugar at medium-high speed until creamy, 3 to 4 minutes, stopping to scrape sides of bowl. Reduce mixer speed to medium, and add egg yolks and vanilla bean paste. Reduce mixer speed to low. Add flour, ¾ cup chopped nut mixture, remaining ½ teaspoon (1.5 grams) salt, and remaining ½ teaspoon (1 gram) nutmeg. Increase mixer speed to medium, and beat just until mixture comes together, 2 to 3 minutes.
Roll dough into 1-inch balls. Roll balls in remaining nut mixture to coat, and place 2 inches apart on prepared pans. Using your thumb or the back of a spoon, gently make an indentation in center of each ball.
Bake for 10 minutes; remove from oven, and press down centers again. Fill centers of cookies with desired jam. Return to oven, and bake until lightly browned, 3 to 5 minutes more. Let cool on pans for 2 minutes. Remove from pans, and let cool completely on wire racks.
Recipe by Bake from Scratch at https://bakefromscratch.com/nutty-thumbprint-cookies-with-jam/