For softer cookies, bake 12 minutes; for crisper ones, bake 16 minutes. If you'd like, melt extra chocolate for the filling, then add to cookie centers and top with the jam.
Preheat oven to 350 degrees, with racks in upper and lower thirds. Place dough in the bowl of an electric mixer fitted with the paddle attachment. Set a small heatproof bowl over a pan of simmering water; add the chocolate, and stir until melted and smooth. Drizzle chocolate over dough; mix on low speed until just combined.
Place walnuts in a shallow dish. Form dough into 1-inch balls; roll balls in nuts, coating completely. Place balls on parchment- lined baking sheets. Use your thumb to make an indentation in center of each ball. Chill 20 minutes.
Bake cookies in upper and lower thirds of oven, switching positions of sheets halfway through, until firm to the touch, 12 to 16 minutes. Remove from oven. Using the handle of a wooden spoon, press indentation of each cookie to redefine shape; fill each with about 1/4 teaspoon jam. Transfer sheets to wire racks to cool completely. Store in airtight containers at room temperature, up to 5 days.