TO MAKE THE COOKIES:

Combine the hazelnuts, nibs, flour, cinnamon, and cocoa powder in a medium bowl, and set aside.

In the bowl of a stand mixer, fitted with the paddle attachment, combine the butter, zest, and lemon juice and mix on medium speed. Add the sugar and mix on low speed for 30 seconds to blend, then increase the speed to medium and beat until fluffy, 2 to 3 minutes. Add the dry ingredients and mix on low speed, stopping to scrape down the sides of the bowl, until just incorporated, 2 to 3 minutes.

Form the dough into a 7-inch square, wrap in plastic wrap, and refrigerate for 30 minutes.

Meanwhile, remove the lid from the jar of jam, place the jar in the microwave, and microwave for 1 minute. Stir and continue to heat until the jam is melted and hot. Or place the jam in a small saucepan and heat over medium heat until softened. Strain the jam into a small bowl and set the fruit aside for another use.

Position the racks in the lower and upper thirds of the oven and preheat the oven to 350°F. Line two baking sheets with Silpats or parchment paper.

Cut the dough in half. Roll each piece between two pieces of parchment paper or plastic wrap until 3/16 inch thick. Cut out cookies with a 1½- to 2-inch square cutter or a knife and place on the prepared sheets about ½ inch apart.

Bake for 18 to 20 minutes, rotating the sheets halfway through baking. Slide the Silpats or parchment with the cookies onto cooling racks to cool completely.

Place half the cookies upside down on a work surface. Top each with about 1 teaspoon of jam. Place the remaining cookies right side up over the filling, and push down gently to sandwich the cookies.

TO COAT THE COOKIES IN CHOCOLATE:

Line a baking sheet with parchment paper.

Dip half of each cookie in the chocolate and place on the baking sheet to harden.