1 cup unsalted butter at room temperature, cut into pieces
1/4 teaspoon almond extract or vanilla extract
For topping and assembly:
1/2 cup confectioners' sugar
4 ounces semisweet chocolate melted
3/4 cup Nutella chocolate-hazelnut spread
Put the flour, nuts, sugar and salt in the bowl of a food processor fitted with the metal blade attachment and process until the nuts are finely ground. Turn the machine on, add the butter a few pieces at a time through the feed tube, and process until evenly combined.
Pulse in the almond extract and run the machine until the mixture begins to form large, moist clumps. Form into two very flat disks, wrap in plastic wrap, and refrigerate for at least 2 hours or until firm enough to roll. The dough may be refrigerated overnight or frozen for up to 1 month; defrost in the refrigerator overnight before proceeding.
Position racks in the upper and lower third of the oven. Preheat the oven to 325. Line two jelly roll pans with parchment paper and set aside.
Remove one disk from the refrigerator and roll out to a 1/4 inch thickness between two very lightly floured pieces of parchment paper. Cut out large cookies and place on prepared cookie sheet at least 1 inch apart. Cut out a small cookie from the center of half the cookies. If you like, you can bake these smaller cookies together on a third baking sheet but their baking time will be less.
Repeat with the remaining dough. You will need to reuse the pans a few times. Take care to make sure you have the same number of solid cookies bottom and cut out cookie tops.
Bake for 10 to 12 minutes for small cookies and 20 to 25 minutes for the larger cookies or just until the cookies turn light brown around the edges. Cool completely on the pans set on racks.
Dust the tops of the cookies with confectioners' sugar or use a parchment cone or plastic ziploc with the tip cut out and make zigzags back and forth over the cookies. Allow the chocolate to firm up.
Use a small offset spatula to spread a thin layer of nutella on the flat bottoms of large solid cookies. Carefully sandwich the decorative cutout cookie top with the cookie bottoms.