Linzer cookies just might be my favorites -- what's not to like about two buttery cookies sandwiched around a sweet filling? Some form or another of the recipe has been around for over 300 years so I decided it was time to give it an update with some of my favorite flavors of all-time -- chocolate and hazelnut.
Add the nuts to the bowl of a small food processor fitted with a blade and pulse until finely ground (it should look like meal and be powdery). Set aside.
Don't overprocess the nuts or they will become hazelnut butter!
Combine the ground hazelnuts, flour, cinnamon, baking powder, and salt in a large bowl and whisk until aerated and any lumps are broken up.
Combine the butter and sugars in the bowl of a stand mixer fitted with a paddle and mix on medium speed until airy and light, about 3 minutes. Add the egg yolks and vanilla and mix until well combined.
Turn off mixer, scrape down the sides, and add the flour mixture. Mix on low until just combined. Divide the dough in two pieces, place each in a square of plastic wrap or parchment, pat into a square that's 1-inch thick, and wrap tightly. Place in the refrigerator to chill, at least 1 hour.
The dough can be made up to 1 month in advance and stored in an airtight plastic bag or container in the freezer.
When ready to bake, heat the oven to 350°F and arrange racks in the upper and lower thirds. Remove the dough from the refrigerator and let warm up until easy to roll and pliable, at least 10 minutes.
Place half the dough on a lightly floured piece of parchment or wax paper, and, using a lightly floured rolling pin, roll to a 1/4-inch thickness. Using a 2-inch cookie cutter, stamp out cookies. Using a second smaller cookie cutter, stamp out the centers of half of the cookies to make stars or rings.
Arrange cookies on a parchment-lined baking sheet and freeze briefly, at least 5 minutes, before baking. Gather dough scraps into a ball and refrigerate until ready to use.
Bake 2 sheets of cookies at a time until the undersides are golden and the tops are set, switching the sheets halfway through baking, about 12 to 15 minutes total. Transfer cookies to a cooling rack then repeat with remaining dough to bake off all the cookies.
To assemble, spread 1 teaspoon of the chocolate-hazelnut spread on the solid cookie then top it with a ring or star-shaped cookie. Repeat to assemble all the linzer cookie sandwiches.