01 -
In a medium bowl, whisk together the flour, cocoa powder, baking powder, and sea salt. Set aside.
02 -
Using a stand mixer fitted with the paddle attachment or a large mixing bowl, beat the butter with brown and granulated sugars on medium speed until light and fluffy, about 2 to 3 minutes.
03 -
Add egg yolk, vanilla extract, and milk to the creamed mixture. Beat until smooth, scraping the bowl sides as necessary.
04 -
On the lowest mixer speed, add the dry ingredient mixture in two additions, mixing just until combined. If dough remains crumbly, press together gently by hand to form a smooth ball.
05 -
Cover the bowl with plastic wrap and refrigerate for at least 1 hour or overnight for better texture.
06 -
Remove dough from fridge, letting it soften at room temperature for 10 to 15 minutes. Scoop 1 tablespoon portions and roll each into a smooth ball. Coat each ball in finely chopped hazelnuts.
07 -
Arrange the hazelnut-coated dough balls on parchment-lined baking sheets. Press a thumb or spoon handle indentation into the center of each.
08 -
Place the trays back into the refrigerator for 30 minutes to ensure the cookies maintain their shape during baking.
09 -
Preheat oven to 350°F. Bake for 10 to 12 minutes until edges are set but centers remain slightly soft. If indentations puff up, gently press them down while cookies are warm. Cool on baking sheet for 5 minutes before transferring to a wire rack to cool completely.
10 -
Place Nutella in a heatproof bowl. Heat heavy cream in a small saucepan just until simmering, then pour over Nutella. Let sit 5 minutes, then stir until smooth and creamy.
11 -
Transfer ganache to a piping bag, snip the tip, and fill the centers of each cooled cookie with ganache. Allow ganache to set at room temperature before serving.