For the walnut rings, quickly knead all ingredients together. The butter should be cold – as well as the hands. Sift the powdered sugar. Wrap in plastic wrap and refrigerate for 2 hours.
Preheat oven to 170 °C and line a baking sheet with baking paper. On a floured work surface, roll out the dough. Using a ring cutter, cut out large cookies.
Cut out a smaller ring in the middle of half of the rings. Place the cookies on the baking sheet. Whisk the egg yolk and brush the rings with it.
Spread the grated walnuts on top. Bake the cookies for 10-12 minutes until light. Warm the jam a little, stir until smooth and put one ringlet with a hole and one without a hole together.