Step 1: Sift the almond flour, hazelnut flour, powdered sugar, and cocoa powder together twice into a large bowl to ensure a smooth batter.
Step 2: In a clean stand mixer bowl, whisk egg whites and cream of tartar on low speed until foamy, then gradually add granulated sugar while increasing speed to medium-high.
Step 3: Continue whisking until stiff, glossy peaks form that do not move when the bowl is inverted.
Step 4: Fold the dry ingredients into the meringue in three additions using a rubber spatula, employing the ‘macaronage’ technique until the batter flows like slow lava.
Step 5: Transfer the batter to a piping bag with a round tip and pipe 1.5-inch circles onto parchment-lined baking sheets.
Step 6: Tap the trays firmly on the counter to release air bubbles and let the shells rest for 30-45 minutes until a dry skin forms on top.
Step 7: Bake at 300°F (150°C) for 14-16 minutes. Allow to cool completely on the tray before removing.
Step 8: For the filling, heat heavy cream until simmering, pour over chopped chocolate and Nutella, let sit for 2 minutes, then stir until smooth. Chill until thickened.
Step 9: Pipe a ring of ganache onto a shell, place a whole hazelnut in the center, and sandwich with a second shell.
Step 10: Drizzle with melted chocolate and sprinkle with crushed hazelnuts. Refrigerate for 24 hours to mature before serving.