Using an electric mixer preferably fitted with the paddle attachment, mix the flour, powdered sugar, cornstarch and salt in a large bowl to blend. Add the butter and mix on low speed until dough forms. Stir in 1/2 cup finely diced walnuts. Roll the dough into an even 10-inch log, and roll in coarsely chopped toasted walnuts. Wrap the log in waxed paper and refrigerate until firm, about 2 hours. (Can be prepared up to 4 days ahead.) Preheat the oven to 350°F. Line 2 heavy large baking sheets with parchment. Using a long sharp knife, slice the dough into thin rounds. Arrange the rounds on prepared baking sheets, spacing evenly. Bake until golden brown, about 10 minutes. Spoon a teaspoon of jam onto half of the cookies. Return the cookies to the oven and continue baking until the jam just begins to bubble at the edges, about 2 minutes. Remove the cookies from the oven. Place a plain cookie atop cookies with jam, forming a sandwich. Cool completely. Arrange cookies on the parchment-lined baking sheets. Dip a fork into the melted chocolate and gently wave the fork over the cookies, allowing the chocolate to drizzle over the cookies in a decorative pattern. Immediately sprinkle the cookies with remaining coarsely chopped nuts. Chill until chocolate sets. Store the cookies in an airtight tin up to 3 days.