Preheat oven to 350F. Place ½ cup plus 2 tablespoons hazelnuts (those needed for the cookies and what's needed for topping on a baking sheet. Toast hazelnuts 10-12 minutes, or until golden and fragrant.
Wrap the toasted hazelnuts in a kitchen towel and rub to remove skins. Let cool completely.
For Coating:
In a small bowl, combine sugar and cinnamon. Stir to combine. Set aside.
For Cookie Dough:
Place ½ cup toasted (and cooled!) hazelnuts in the bowl of a food processor and process until finely ground.
In a large bowl, combine ground hazelnuts, flour cocoa powder, baking soda, salt, and cinnamon. Stir to combine. Set aside.
In the bowl of a stand mixer, combine butter and sugar. Beat until light and fluffy. Scrape down the sides of the bowl.
Add egg, vanilla extract, and chocolate extract. Mix to combine, then scrape down the sides of the bowl again.
With the mixer running on low speed, gradually add the dry ingredients just until the dough comes together.
Using a medium cookie scoop (about 2 tablespoons each; see note), portion dough onto cookie sheets lined with parchment paper or silpats (see note). Using your palms, roll each portion into a ball and roll in the cinnamon/sugar coating. Dough balls should be placed on the cookie sheets at least 2" apart.
Bake 10-12 minutes, rotating the baking sheets halfway through. Cookies will be puffed.
Remove cookies from the oven and immediately use a rounded measuring spoon to create the "thumbprint" in the center of each cookie. Allow cookies to cool on the baking sheet 10 minutes, then remove to a wire rack to cool completely.
For Ganache:
In a medium heatproof bowl, combine butter and chocolate. Microwave on 30-second intervals, stirring after each interval, until mixture is smooth.
Pour ganache into a zip-top bag and snip the corner. Pipe ganache into the center of each of the cooled cookies
For Topping:
Immediately top the ganache with chopped toasted hazelnuts. Once the ganache is set, drizzle the tops of the cookies with the melted chocolate. Let chocolate set at room temperature.Enjoy!
Notes
Note on hazelnuts: I can rarely find hazelnuts at my local grocery store. If you can't find them, they're online [here].Note on chocolate extract: I highly recommend using chocolate extract in the recipe! This (sometimes overlooked) extract lends an amazing depth of flavor to chocolate baked goods! If you can’t find it in your local grocery store, you can find some [here]. Even better, makes yourself some [Homemade Chocolate Extract] for next time!Note on cookie scoop: I used my medium cookie scoop (this one [here]) to portion out these cookies. They turned out a nice size and gave me 36 cookies in all.Note on silpats: I like using silpats to line my baking sheets for cookies because nothing sticks to them and I can use them over and over again (as opposed to parchment paper, which ends up in the garbage). If you're looking to get yourself some silpats, you can find them [here].