Preheat oven to 350 degrees

Prepare a 15½-by-17-inch jelly-roll pan. Combine the flour, baking powder and salt. Stir in the sugar, walnuts, butter and sour cream. The dough will be soft. Dust your hands with flour and pat the dough into the jelly-roll pan. Refrigerate it for 10 minutes. Bake it for 20 minutes and then cool for 10 minutes before filling. While the dough is baking, make the filling by grinding all the ingredients together until somewhat pasty. Do this job in a food processor or with a meat grinder.

Cut half of the dough into 20 squares, leaving the other half uncut. Spread the uncut side with filling. Place the cut squares on top of the filling and cut through to the bottom, forming individual sandwiches. Keep in an airtight container.