Preheat the oven to 350 degrees, and line a heavy large baking sheet with parchment paper.
Whisk the flour, baking powder, cocoa powder and instant coffee in a medium bowl to blend.
Using an electric mixer, beat the sugar, butter, salt and vanilla in a large bowl to blend. Beat in the eggs 1 at a time. Add the flour mixture and beat just until blended. Stir in the hazelnuts and chocolate chips.
Form the dough into a 12-inch long, 4-inch wide log on the prepared baking sheet. Bake until a toothpick inserted into the middle comes out clean, about 35 minutes. Cool for 15 minutes.
Place the log on a cutting board. Using a sharp serrated knife, cut on a diagonal into ½ to ¾-inch-thick slices. Arrange the biscotti, cut side down, on the baking sheet. Bake the biscotti until crisp & dry, about 15 minutes. Transfer the biscotti to a rack and cool completely.
For the glaze, stir the chocolate in a bowl set over a saucepan of simmering water until the chocolate melts. Spread a thin layer of melted chocolate onto one end of each biscotti and immediately sprinkle with the chopped toasted hazelnuts. Do this over the sink or over a second cookie sheet to avoid a mess!
Place the biscotti on the baking sheet for the chocolate to set. Refrigerate until the chocolate is firm, about 35 minutes.