Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper and set aside.
In a medium bowl, whisk together the flour, cocoa powder, kosher salt, and baking powder. Set aside.
In a large mixing bowl, using a hand mixer or stand mixer fitted with the paddle attachment, beat the butter until smooth and creamy.
Add light brown sugar and granulated sugar to the butter. Beat on medium speed until light and fluffy, about 2–3 minutes.
Add the egg yolks and vanilla extract. Mix until fully incorporated.
Add the dry ingredients to the wet ingredients in two additions, mixing on low speed until the dough just comes together. Do not overmix.
Scoop out tablespoon-sized portions of dough and roll into smooth balls using your hands. Place onto the prepared baking sheets, spacing them about 2 inches apart.
Use your thumb or the back of a ½ teaspoon measuring spoon to gently press an indentation into the center of each dough ball.
Bake for 10–12 minutes, or until the cookies are set around the edges but still soft in the center. If the indentations puff up during baking, gently press them down again right after removing from the oven.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Once the cookies are fully cooled, spoon a heaping teaspoon of Nutella into the center of each one.
Sprinkle chopped toasted hazelnuts over the Nutella while it’s still soft so they stick.
Let the filling set before serving or storing.