01 -
In a medium bowl, add the flour, cocoa powder, baking powder, and salt. Whisk together and set aside.
02 -
In a large mixing bowl or stand mixer fitted with the paddle attachment, beat the butter and sugars on medium speed until light and fluffy, about 2-3 minutes.
03 -
Add the egg yolk, vanilla, and milk. Beat until smooth, scraping down the sides of the bowl as needed.
04 -
With the mixer on the lowest speed, add the dry ingredients in two additions. Mix until just combined. Use your hands to press the dough into a smooth ball as it will appear crumbly.
05 -
Cover the bowl with plastic wrap and refrigerate for at least 1 hour, or overnight for better results.
06 -
Let the dough sit at room temperature for 10-15 minutes. Scoop 1 tablespoon portions (about 20g each), roll into balls, and coat in chopped hazelnuts. Place on a parchment-lined baking sheet and press indentations into the center of each cookie with your thumb or the handle of a wooden spoon.
07 -
Refrigerate the shaped cookies for another 30 minutes to help them retain their shape.
08 -
Preheat the oven to 350ºF. Bake the cookies for 10-12 minutes, until the edges are set and the centers slightly soft. Press indentations once more if they puff up. Cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
09 -
Add Nutella to a heatproof bowl. Heat the cream in a small saucepan until just simmering but not boiling. Pour over the Nutella, let sit for 5 minutes, then stir until smooth and creamy.
10 -
Transfer the ganache to a piping bag and snip off the tip. Fill the cookie centers with ganache and allow to set at room temperature.