2 cups all-purpose flour
1 teaspoon baking powder
1/4 teaspoon salt
4 tablespoons unsalted butter, softened
1 cup sugar
2 large eggs
1/2 teaspoon vanilla extract
3/4 cups finely chopped hazelnuts (with a food processor)
1 cup dark cooking chocolate
Preheat oven to 350 degrees F. Line a baking sheet with parchment paper. Whisk the flour, baking powder, and salt together in a medium bowl.
In a large bowl, beat the butter and sugar together until light and fluffy, 3 to 6 minutes. Beat in the eggs, one at a time, then the vanilla until combined, about 30 seconds, scraping down the bowl and beaters as needed.
Slowly mix in the flour mixture until combined, about 30 seconds. Mix in the hazelnuts until just incorporated.
Press the dough into two 13 by 2-inch loaves on the prepared baking sheet, spaced about 3 inches apart. Bake the loaves until golden and just beginning to crack on top, about 35 minutes, rotating the baking sheets halfway through baking.
Let the loaves cool on the baking sheet for 10 minutes. Lower the oven temperature to 325 degrees F.
Transfer the loaves to a cutting board and slice each on the diagonal into 1/2-inch-thick slices with a serrated knife. Lay the slices about 1/2 inch apart on the baking sheet and bake until crisp and golden brown on both sides, about 15 minutes, flipping the slices over halfway through baking. Transfer the biscotti to a wire rack and let cool completely before serving, about 1 hour.
Melt dark chocolate over a double boiler. Dip the ends or the entire flat side of each biscotti into the melted chocolate. Prop each cookie vertically in the aisles of the cooling rack, so the chocolate faces the ceiling.