Preheat oven to 350 degrees with rack in lower third. Place hazelnuts in a food processor; process until as fine as possible, about 1 minute.
Add confectioners' sugar; process until combined, about 1 minute.
Pass hazelnut mixture through a fine-mesh sieve. Transfer solids in sieve to food processor; grind and sift again, pressing down on clumps. Repeat until less than 2 tablespoons of solids remains in sieve.
Whisk egg whites and granulated sugar by hand to combine. Beat on medium speed (4 on a KitchenAid) 2 minutes. Increase speed to medium-high (6) and beat 2 minutes. Then beat on high (8) 2 minutes more.
The beaten egg whites will hold stiff, glossy peaks when you lift the whisk out of the bowl. Beat on highest speed 30 seconds.
Add dry ingredients all at once. Fold with a spatula from bottom of bowl upward, then press flat side of spatula firmly through middle of mixture. Repeat just until batter flows like lava, 35 to 40 complete strokes.
Rest a pastry bag fitted with a 3/8-inch round tip (Ateco #804) inside a glass. Transfer batter to bag; secure top. Dab some batter remaining in bowl onto corners of 2 heavy baking sheets; line with parchment.
With piping tip 1/2 inch above sheet, pipe batter into a 3/4-inch round, then swirl tip off to one side. Repeat, spacing rounds 1 inch apart. Tap sheets firmly against counter 2 or 3 times to release air bubbles.
Heat heavy cream in a saucepan until bubbles begin to form. Add chocolate and butter; stir to combine. Stir in chocolate-hazelnut spread. Let cool until thick and spreadable.
Bake 1 sheet at a time, rotating halfway through, until risen and just set, 13 minutes. Let cool. Pipe or spread filling on flat sides of half of cookies; top with remaining half. Wrap in plastic and refrigerate.
Add in as needed!
Preheat oven to 375 degrees. Whisk whites with a mixer on medium speed until foamy. Add cream of tartar, and whisk until soft peaks form. Reduce speed to low, then add superfine sugar. Increase speed to high, and whisk until stiff peaks form, about 8 minutes. Sift flour mixture over whites, and fold until mixture is smooth and shiny.
Transfer batter to a pastry bag fitted with a 1/2-inch plain round tip, and pipe 3/4-inch rounds 1 inch apart on parchment-lined baking sheets, dragging pastry tip to the side of rounds rather than forming peaks. Tap bottom of each sheet on work surface to release trapped air. Let stand at room temperature for 15 minutes. Reduce oven temperature to 325 degrees. Bake 1 sheet at a time, rotating halfway through, until macarons are crisp and firm, about 10 minutes. After each batch, increase oven temperature to 375 degrees, heat for 5 minutes, then reduce to 325 degrees.
Let macarons cool on sheets for 2 to 3 minutes, then transfer to a wire rack. (If macarons stick, spray water underneath parchment on hot sheet. The steam will help release macarons.)
Sandwich 2 same-size macarons with 1 teaspoon filling. Serve immediately, or stack between layers of parchment, wrap in plastic, and freeze for up to 3 months.