In a food processor, finely grind the walnuts and cake with the sugar. Transfer the mixture to the bowl of an electric mixer.
Add the softened butter to the ground nut and sugar mixture. Using the paddle attachment, blend the mixture on medium speed.
Add the egg and blend well.
In a small bowl, whisk together the flour, baking powder, and cinnamon.
On low speed, add the dry ingredients into the butter and nut mixture. Blend well.
Wrap the dough in plastic wrap and chill it in the refrigerator for at least 3 hours to firm up.
Assembling the Linzer: Preheat the oven to 325°F.
Spray 6-8 4 ½ inch diameter false bottom tart pans with nonstick cooking spray, and place on 1 or 2 aluminum foil-lined sheet pans.
Crumble some of the firm dough into each 4 ½ inch diameter false bottom tart pan, patting it evenly over the bottom and up the sides. The dough should be approximately ¼ inch thick.
Spread some of the jam into each tart until the tart is ¾ of the way full.
Roll out some of the remaining dough into a small rectangle, at least 4 ½ inches wide, on a lightly floured work surface.
Using a pizza cutter, cut the dough into long strips approximately ¼ inch thick.
Place approximately 4 strips across the filling on each tart, evenly spacing them out, and trimming the edges to be flush with the tart pan.
Cut another 4 strips of dough and place them 45° across the other strips forming diamond openings as the strips crisscross each other. Trim the ends to be flush with the tart pan.
Place the tarts on the prepared sheet pans.
Bake for approximately 30-45 minutes or until the crust is brown and the jam is bubbly.
Cool and remove tarts from pans.
Serve warm or at room temperature, dusted with confectioners' sugar.
Notes
Makes 5-6 4 ½ inch diameter tortes (the number of tarts will depend on the size of your tart pans)