Place 80g (about ⅛ cup) walnuts in a food processor and pulse until coarsely chopped. Do not grind into powder or overprocess to avoid releasing oil. Set aside.
Soften 180g unsaltedbutter to room temperature — it should yield easily when pressed.
Add the butter to amixing bowl and lightly cream. Add 80g powdered sugar and mix until smooth,about 50 strokes, or until the mixture lightens in color.
Mix in ¼ tsp salt and1 tsp vanilla extract until well combined.
Add 180g cake flourand stir until no dry flour remains.
Place the dough in aplastic bag or wrap in cling film. Flatten slightly and refrigerate for 30minutes until firm.
After chilling, prepare a scale, the chopped walnuts, and a baking sheet.
Divide dough into 20g portions, roll into balls, and coat the surface with walnuts. Place on the baking sheet, leaving space between each.
Gently press each ball into a thick disk, keeping enough height for the jam filling.
Press a small indentation in the center with your thumb (or a tool). Fill a piping bag with jam (strawberry, blueberry, or your favorite), snip the tip, and pipe jam into each indentation.
Bake at 180°C (356°F) for 13–15 minutes.
Transfer cookies to a wire rack to cool completely.
Enjoy your crisp, nutty, and fruity thumbprint cookies!