Preheat oven to 350°F. Line baking sheets with parchment paper.
Toast hazelnuts 3–4 minutes until fragrant; chop when cooled.
Cream butter, brown sugar, and granulated sugar until fluffy.
Beat in eggs, vanilla, and Nutella. Mix in milk.
Whisk flour, cocoa, baking soda, and salt separately.
Add dry ingredients to wet; fold in hazelnuts and chocolate.
Scoop 1.5-inch dough balls onto sheets, 2 inches apart.
Bake 10–12 minutes until edges set, centers soft.
Cool 5 minutes on sheet, then transfer to rack.