Nutella crinkle cookies

Toast the hazelnuts if desired by spreading them on a lined baking sheet and warming in a 300°F oven until lightly golden, then let cool briefly.
Whisk the flour, cocoa powder, baking soda, and salt together in a medium bowl until evenly blended.
Beat the softened butter with both sugars on medium-high speed until pale, fluffy, and airy in texture.
Blend in the egg, egg yolk, and vanilla, then beat in the Nutella until smooth and fully incorporated.
Mix the dry ingredients into the dough on low speed just until no dry streaks remain, then gently blend in the hazelnuts.
Cover the bowl tightly and refrigerate the dough for at least 2 hours until firm and scoopable.
Preheat the oven to 350°F and line baking sheets with parchment paper, setting up bowls of granulated and powdered sugar for rolling.
Scoop chilled dough into small portions, shape into shallow cups, fill half with Nutella, seal with remaining dough, and roll into smooth balls.
Roll each dough ball first in granulated sugar, then generously coat with confectioners’ sugar before spacing on baking sheets.
Bake until edges look set but centers remain soft, then remove from the oven.
Let cookies rest briefly on the pan before transferring to a rack to cool completely, adding extra powdered sugar if desired.