Chocolate Hazelnut Thumbprint Cookies

Preheat oven to 350º. On a rimmed baking sheet, roast hazelnuts until fragrant and noticeably darker, 10 to 12 minutes. Let cool slightly. Transfer hazelnuts to a food processor and process until finely ground.  Meanwhile, in a medium bowl, whisk flour, cocoa powder, baking soda, and salt. Whisk in ground hazelnuts. Combine chocolate and 4 tablespoons butter in a microwave-safe bowl. Microwave in 15 second bursts, stirring after each burst, until melted, about 1 minute. Set aside to cool slightly. In a large bowl using a hand mixer or a stand mixer using the paddle attachment, beat brown sugar, ⅓ cup granulated sugar, and remaining butter until slightly fluffy, about 2 minutes. Add egg, vanilla, and milk, and beat for 1 minute. Add the melted chocolate mixture and beat until well-combined, 1 minute more. With the mixer on low, beat in flour until evenly combined.  Line two large baking sheets with parchment paper. Spread remaining ⅓ cup granulated sugar on a plate. Scoop out level tablespoons of dough, dropping mounds on prepared baking sheets as you go. Roll each mound into a ball, then roll in the sugar and place on prepared baking sheets, 2 inches apart. Gently press your thumb into each ball.  Freeze raw cookies until firm, at least 1 hour and up to 2 days.  Preheat oven to 350° with racks in the lower and upper third positions. Bake, rotating sheets halfway, until slightly puffed, 7 to 8 minutes. Once out of the oven, reinforce the indentation of the cookie by gently pressing again with a brave thumb.  Microwave Nutella until slightly runny but not completely liquid, about 20 seconds. While cookies are still warm, spoon ½ teaspoon of Nutella into each indentation.