PEELING AND ROASTING HAZELNUTS: Bring 2 cups of water to a boil in a medium saucepan over medium-high heat. Add 2 tbsp. baking soda, and 1½ cups unpeeled hazelnuts. Boil for 3 minutes (the water will turn black). Drain in a colander. Transfer nuts to a large bowl of cold water and slip skins from nuts with your fingers. Dry nuts on paper towels, then place on a baking sheet and roast in a preheated 350° oven until golden, 15-20 minutes. Remove from oven and set aside to cool completely.
GRINDING NUTS: Set aside 1½ cups peeled roasted hazelnuts and 1½ cups blanched sliced almonds. Working with one kind of nut at a time—and in small batches, to avoid grinding nuts into nut butter—put a small handful into an electric coffee grinder and pulse nuts into a fine powder. Repeat process until all nuts are ground. Transfer to a bowl for use in dacquoise.