Prep the oven and pans: Start by preheating your oven to 350°F. Line a baking sheet with parchment paper so the cookies release easily and bake evenly. Set it aside while you work on the dough.
Mix the dry ingredients: In a small bowl, stir together the flour and salt. Mixing them early helps spread the salt evenly through the dough later.
Cream butter and sugar: In a larger bowl, beat the softened butter with the brown sugar until the mixture looks light and fluffy. This step builds the cookie’s soft texture and rich flavor.
Add egg yolk and vanilla: Mix in the egg yolk and vanilla extract until fully combined. Save the egg white in a small bowl since you will use it shortly for coating.
Form the dough: Gradually add the flour mixture to the butter mixture, stirring just until a soft dough forms. It should hold together easily without feeling sticky.
Shape and coat: Roll the dough into small balls, about one tablespoon each. Dip each ball lightly into the egg white, then roll it in the chopped walnuts until coated.
Create the thumbprint: Place the coated dough balls on the baking sheet. Use your thumb or the back of a spoon to gently press an indentation into the center of each cookie.
Fill with jam: Spoon a small amount of raspberry jam into each indentation. Do not overfill since the jam will spread slightly while baking.
Bake the cookies: Bake for 12 to 15 minutes until the edges look set and lightly golden. Remove from the oven and let them cool on the pan for a few minutes before transferring to a rack.
