Making the linzer dough

• Combine flour, salt and baking powder in a medium bowl.
• Finely grind hazelnuts with 2/3 cup sugar in food processor.
• In the bowl of a stand mixer beat butter until light and fluffy. 
• Add ground nuts and sugar mixture, beating to blend.
• Add whole egg, egg yolk, and vanilla extract and blend just until incorporated. With a rubber spatula, stir in flour mixture, scraping down sides of bowl.
• Divided dough into 4 equal portions. Flatten into disks and wrap in plastic wrap. Refrigerate until firm, about 2 hours. (The dough can be frozen for up to 2 months at this point)
• Preheat oven to 325 degrees. Roll one dough disk out on a heavily work surface to about 1/8″ thick. These are sandwich cookies so they should be quite thin.
• Using a cookie cutter that’s about 2″ to 3″ in diameter and cut out as many cookies as possible from the dough. Transfer cookies to an un-greased cookie sheet with parchment paper or silpat. Using a small pastry bag tip cut out holes on half of the cookies.
• Gather up the dough scraps and press into a smaller disk and place in the fridge to chill.
• Bake cookies on prepared cookie sheets until golden brown, about 10 minutes. Transfer baked cookies to a wire rack to cool. Repeat with remaining dough and scraps.
• While cookies are baking and cooling, prepare the jam filling. Boil jam in a heavy small saucepan until thickened, stirring constantly, about 4 minutes. 
• Dust the top half of the cookies with powdered sugar using a fine mesh sieve or this powdered sugar wand.
• Once cookies are cool, spread about 1 teaspoon on the bottom cookies, leaving a 1/4″ border. Match each cookie with a cut-out cookie top.
• Cookies can be made up to 2 weeks in advance. Place cookies in an airtight container with parchment paper separating the layers.