Ingredients

• 1/2 cup plus 2 tablespoons hazelnuts (2.5 ounces)
• 2 cups all-purpose flour
• 1/2 cup unsweetened cocoa powder
• 1/2 teaspoon baking soda
• 1/2 teaspoon kosher salt
• 2 1/4 teaspoons ground cinnamon, divided
• 1 cup granulated sugar
• 1 1/4 cups (2 1/2 sticks) unsalted butter, room temperature, divided
• 1 large egg
• 1 tablespoon vanilla extract
• 1/2 cup sanding or pure cane sugar
• 4 ounces semisweet chocolate, chopped (about 1 cup)
• A spice mill

Instructions

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.
2. Grind 1/2 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.
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.
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.
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.
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 1/4" hole in one corner. Pipe chocolate into cookie indentations. Immediately sprinkle chocolate with reserved nuts. Let cookies stand until chocolate sets, about 20 minutes.
7. Do Ahead Cookies can be made up to 1 week ahead; keep in an airtight container at room temperature.
8. Cookies can be made up to 1 week ahead; keep in an airtight container at room temperature.