⅓ cup chocolate-hazelnut spread (such as Nutella®)
1 ¼ cups bittersweet chocolate chips
1 cup chopped raw hazelnuts
½ cup milk chocolate candy wafers
Preheat the oven to 325 degrees F (165 degrees C). Position racks in top and bottom third of the oven. Line two large rimmed baking sheets with parchment paper and lightly spray with cooking spray.
Whisk flour, cocoa powder, baking powder, and salt together in a medium bowl.
Beat eggs in the bowl of a stand mixer fitted with a whisk attachment on medium-high speed until pale and frothy, about 2 minutes. Gradually add sugar and mix until pale and thickened, about 2 minutes. Beat in melted butter and vanilla on low speed until combined.
Add chocolate-hazelnut spread and beat on low speed, scraping down sides and bottom of bowl as needed, until just combined, about 30 seconds. Gradually mix in flour mixture and on low speed until just combined, about 30 seconds. Fold in chocolate chips and hazelnuts until just combined.
Divide dough in half. Place each half in the center of a prepared baking sheet. Lightly spray your hands with cooking spray and shape each piece of dough into a 3/4-x2 1/2-x13-inch log.
Bake in the preheated oven, rotating cookie sheets halfway through, until set and firm to the touch, about 30 minutes. Remove from the oven and let cool on the baking sheets for 10 minutes. Reduce oven temperature to 300 degrees F (150 degrees C).
Carefully transfer one log to a cutting board. Use a serrated knife to cut 1/2-inch slices on the diagonal. Transfer slices back to the baking sheet in an even layer, with cut-sides up. Repeat with the remaining log.
Bake in the preheated oven until dried, about 30 minutes, flipping cookies and rotating baking sheets halfway through. Transfer biscotti to a wire rack and let cool completely, about 30 minutes.
Meanwhile, microwave candy wafers in a medium microwave-safe bowl on high power for 30 seconds, then stir. Continue microwaving and stirring in 30-second increments, until melted and smooth, about 1 to 1 1/2 minutes total. Drizzle melted chocolate over one end of each biscotti.
Refrigerate, uncovered, until chocolate hardens, about 15 minutes.