In heat-proof measuring cup heat milk in microwave on medium power for 30 seconds, or until temperature reaches 90-100 degrees F. Stir in cocoa powder with fork, set aside until ready to use.
In large bowl whisk together flour, baking powder, and salt. In bowl of stand mixer, beat together Nutella, butter, and sugar on medium-high speed until light and fluffy, about 1-2 minutes. Add eggs and vanilla and mix until combined. Turn mixer off and add flour mixture and cocoa powder mixture; pulse mixer on and off until almost combined. Increase speed to low and mix until fully combined, about 30 seconds. Fold in ½ cup hazelnuts, and chill until firm, about 60 minutes.
After cookie dough has chilled for 30 minutes, adjust oven racks to upper-middle and lower-middle positions and heat oven to 375 degrees F. Line 2 cookie sheets with parchment paper.
Once dough has chilled for 60 minutes, place remaining hazelnuts in a pie plate, and powdered sugar into another pie plate. Using 1 ½ tablespoon scoop, scoop dough one mound at a time. Roll dough into ball, and roll into hazelnuts (dough should be studded with hazelnuts, not covered). Roll dough balls lightly in your hands to compress hazelnuts into dough. Next, roll into powdered sugar and place balls on prepared cookie sheets, 2-inches apart. Bake until set, about 12-14 minutes, switching and rotating pans halfway through baking. Cool cookies on cookie sheets for 5 minutes, then move to wire racks to cool completely. Repeat with remaining dough.