These decadent cookies make the most of their headlining ingredients. The hazelnuts are toasted, which deepens their sweetness and makes them the perfect complement to a luscious dark chocolate and cinnamon pairing.

　　

Ingredients

Makes about 2½ dozen

　　½ cup plus 2 tablespoons hazelnuts (2.5 ounces)

　　2 cups all-purpose flour

　　½ cup unsweetened cocoa powder

　　½ teaspoon baking soda

　　½ teaspoon Diamond Crystal or ¼ teaspoon Morton kosher salt

　　2¼ teaspoons ground cinnamon, divided

　　1 cup granulated sugar

　　1¼ cups (2½ sticks) unsalted butter, room temperature, divided

　　1 large egg

　　1 tablespoon vanilla extract

　　½ cup sanding or pure cane sugar

　　4 ounces semisweet chocolate, chopped (about 1 cup)

　　

Special equipment:

A spice mill

　　

Step 1

Position racks in upper and lower thirds of oven; preheat to 350°F. Toast hazelnuts on a rimmed baking sheet on upper rack, tossing occasionally, until golden brown, 10–12 minutes. Wrap nuts in a kitchen towel and let steam 1 minute. Rub nuts in towel to remove loose skins (don't worry about skins that don't come off) and let cool.

　　

Step 2

Grind ½ cup nuts in spice mill until finely ground. Transfer to a large bowl. Add flour, cocoa powder, baking soda, salt, and 1 1/4 tsp. cinnamon.

　　

Step 3

Using an electric mixer on medium speed, beat granulated sugar and 1 cup butter in another large bowl until pale and fluffy, about 2 minutes. Add egg and vanilla and beat until combined. Reduce mixer speed to low, add dry ingredients, and beat until just combined.

　　

Step 4

Combine sanding sugar and remaining 1 tsp. cinnamon in a small bowl. Roll heaping 1-Tbsp. rounds of dough into balls, then roll in sanding sugar. Arrange balls on parchment-lined rimmed baking sheets at least 2" apart. Bake cookies, rotating sheets halfway through, until puffed and set but still moist, 10-12 minutes.

　　

Step 5

Remove baking sheets from oven and immediately make an indentation in the center of each cookie with the small end of a melon baller or the bottom of a rounded 1-tsp. measuring spoon. Transfer cookies on parchment to wire racks and let cool completely.

　　

Step 6

Finely chop remaining 2 Tbsp. nuts and set aside. Heat chocolate and remaining 4 Tbsp. butter in a small bowl in 10-second intervals in microwave, stirring often, until melted. Transfer chocolate mixture to a small resealable plastic bag and snip a ¼" hole in one corner. Pipe chocolate into cookie indentations. Immediately sprinkle chocolate with reserved nuts. Let cookies stand until chocolate sets, about 20 minutes.

　　Do Ahead: Cookies can be made up to 1 week ahead; keep in an airtight container at room temperature.