Place the nuts and brown sugar in a food processor. Pulse into a very fine crumb. Set aside.

In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and granulated sugar together on medium-high speed until completely smooth and creamy, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg and vanilla extract, and beat on high speed until combined, about 1 minute. Scrape down the sides and bottom of the bowl and beat again as needed to combine.

Add the flour mixture and the ground nut mixture to the wet ingredients and mix on low speed until combined. It will be crumbly at first, but will come together after a couple minutes of mixing.

Divide the dough into 2 equal portions, gently flatten into discs, and wrap each in plastic wrap. Chill the discs in the refrigerator for at least 3 hours and up to 4 days.

Preheat the oven to 350°F (180°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside. If the dough chilled for longer than 3 hours, allow to sit at room temperature for at least 20 minutes before rolling because the dough will be quite hard.

Generously flour a work surface, as well as your hands and rolling pin. The dough may become sticky as you work, so have extra flour nearby to use as needed. Alternatively, the dough may crack when rolling out. If that happens, wait a few minutes for it to soften up, then try again. The more you work with it, the softer and more manageable it will become. Roll out the first disc of dough to 1/4-inch thickness. Using a 2-inch round or fluted cookie cutter, cut the dough into circles. Re-roll the dough scraps and continue cutting until all is used. Repeat with the second disc of dough. You should have about 64 circles.

Using a 1-inch round or fluted cookie cutter, cut a hole into the center of 32 of the circles. Let’s call these 32 cookies the “donut cookies” because of that center hole.

Arrange the whole circles and the donut cookies 2 inches apart and on separate baking sheets (because the donut cookies take 1 less minute to bake).

Bake the whole circles for about 11 minutes, or until lightly browned around the edges, and bake the donut cookies for about 10 minutes. Rotate the baking sheets halfway through the bake time. Allow cookies to cool on the baking sheets for 5 minutes. Using a sieve or sifter, dust the “donut cookies” with confectioners’ sugar, then transfer all of the cookies to a cooling rack to cool completely before assembling.

Spread 1/2 teaspoon of jam on the bottom of each whole cookie. Carefully top each with a donut cookie and press down gently to create a cookie sandwich.

Store tightly covered at room temperature for up to 3 days or in the refrigerator for up to 1 week.