Place the hazelnuts and 1/3 cup (67g) brown sugar in a food processor. Pulse into a very fine crumb. Set aside.

Whisk the flour, baking powder, cinnamon, and salt together in a medium bowl. Set aside.

In a large bowl using a handheld or stand mixer fitted with a paddle attachment, beat the butter and remaining 1/3 cup (67g) brown sugar together on 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 up the bottom of the bowl and beat again as needed to combine. Mixture may look separated or curdled—that’s ok.

Add the flour mixture and ground hazelnut mixture to the wet ingredients and beat on low speed until combined. The dough will be crumbly at first and appear too dry, but it will finally come together after a couple minutes of beating. Yields about 2 lbs of dough.

Divide the dough into 2 equal portions, flatten into 1-inch thick discs, and wrap each in parchment paper or plastic wrap. Chill the discs in the refrigerator for at least 3 hours and up to 4 days. If chilling for longer than 3 hours, allow to sit at room temperature for at least 30 minutes before rolling because the dough will be too hard and crumble. (Time-saving tip: As the cookie dough chills, you can make the caramel filling in step 11. The caramel must cool and thicken before sandwiching.)

Preheat oven to 350°F (180°C). Line baking sheets with parchment paper or silicone baking mats. Set aside.

Remove 1 disc of chilled cookie dough from the refrigerator. Generously flour a work surface, as well as your hands and the rolling pin. The dough may become sticky as you work, so keep extra flour nearby to use as needed. Alternatively, the dough may crack when rolling out. Wait a few minutes for it to soften back up. The more you work with it, the softer it will become. Roll out the disc until 1/4 inch thick. Using a 2 or 2.5-inch cookie cutter, cut dough into shapes. Always be gentle with this fragile dough and work slowly. Reroll the remaining dough and continue cutting until all is used. Repeat with the second disc of dough. You should have about 50-64 cookies, depending on size.

Optional hole in the top cookies: Using a 1-inch cookie cutter, cut a hole into the center of half of the cookies. Let’s call these cookies the “donut cookies” because of that center hole. These will be fragile, so handle with care.

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

Bake the whole cookies for about 11-13 minutes or until lightly browned around the edges, and bake the donut cookies for about 10-12 minutes. Rotate the baking sheets halfway through the bake time. Allow cookies to cool on the baking sheets for 5 minutes. Dust the “donut cookies” with confectioners’ sugar, and then transfer all of the cookies to a wire rack to cool completely before assembling.

Heat granulated sugar in a medium heavy-duty saucepan (avoid using non-stick) over medium heat, stirring constantly with a high heat resistant silicone spatula or wooden spoon. Sugar will form clumps and eventually melt into a thick brown, amber-colored liquid as you continue to stir. Once sugar is completely melted, immediately stir in the butter until melted and combined. Be careful in this step because the caramel will bubble rapidly when the butter is added. If you notice the butter separating or if the sugar clumps up, remove from heat and vigorously whisk to combine it again. (If you’re nervous for splatter, wear kitchen gloves. Keep whisking until it comes back together, even if it takes 3-4 minutes. It will eventually—just keep whisking. Return to heat when it’s combined again.) After the butter has melted and combined with the caramelized sugar, cook for 1 minute without stirring. Slowly stir in 1/2 cup of heavy cream. Since the heavy cream is colder than the hot caramel, the mixture will rapidly bubble when added. After all the heavy cream has been added, stop stirring and boil for 1 minute. Remove from heat and stir in the salt. Cool caramel for at least 3 hours at room temperature. Caramel thickens as it cools.

Spoon 2 heaping teaspoons of caramel onto bottom of whole cookies. Carefully top each with a donut cookie and gently press down to create a cookie sandwich.

Cookies will stay fresh covered at room temperature for 2 days or in the refrigerator for up to 1 week.