Start by roasting the hazelnuts to bring out their rich flavor. Preheat your oven to 350°F (175°C). Spread the whole hazelnuts on a baking sheet and toast them in the oven for about 10 minutes until they are golden and fragrant. Once done, allow them to cool slightly before rubbing off most of the skins. Roughly chop half of the hazelnuts for the cookie dough and set the other half aside for topping.
In a large mixing bowl, cream together the softened unsalted butter, granulated sugar, and brown sugar. Using an electric mixer on medium speed, beat the mixture until it becomes light and fluffy, which should take about 3-5 minutes.
Add the egg and vanilla extract to the butter mixture. Beat again until fully combined, ensuring these wet ingredients enhance the richness of your cookies.
Sift together the all-purpose flour, cocoa powder, baking powder, and salt directly into the wet ingredients. Using a spatula, gently fold the dry ingredients into the mixture until just combined.
Now, fold in the roughly chopped roasted hazelnuts until evenly distributed throughout the dough.
Scoop tablespoon-sized portions of dough and roll them into balls, then flatten each ball into a disc shape. Arrange these discs on a baking sheet lined with parchment paper, ensuring they are spaced about 2 inches apart.
Bake your cookies in the preheated oven at 350°F (175°C) for 10-12 minutes. Keep an eye on them; the edges should be set while the centers remain soft when you remove them from the oven.
While the cookies cool completely on a wire rack, melt the chocolate. Chop your semi-sweet or dark chocolate and place it in a microwave-safe bowl. Heat in 30-second bursts until smooth and fully melted.
Once your cookies are cool, dip half of each cookie into the melted chocolate. Let any excess chocolate drip off before placing them on parchment paper. Immediately sprinkle the finely chopped roasted hazelnuts on top of the chocolate-dipped area.
Allow the chocolate to set at room temperature or chill in the fridge for about 10 minutes until hardened.
