Dutch-process cocoa powder has been treated with an alkaline solution, which removes some of cocoa powder's natural acidity. It's darker and has a smoother, mellower flavor than does its "regular" counterpart.
Preheat the oven to 350 °F (165 °C). Place the hazelnuts on a baking sheet. Bake until light brown, about 8 minutes. Let cool.
Line two large baking sheets with foil and spray the foil with nonstick cooking spray. Adjust the oven racks to divide the oven into thirds.
In a food processor, combine 2 tablespoons of the hazelnuts, the flour, 2 tablespoons of the chocolate chips, the cocoa, espresso powder, baking soda and salt. Process until the nuts are finely ground; transfer the dry ingredients to a large bowl.
In the same food processor (no need to clean), combine the sugar, eggs, egg whites and vanilla until slightly thickened, about 2 minutes. Add to the dry ingredients. Stir in the remaining hazelnuts and chocolate chips.
On one side of the baking sheet, spoon one-fourth of the batter (about 3/4 cup) into a long rope, about 14" long and 1 1/2" wide. Repeat with the remaining batter, making two ropes on each baking sheet, with space in between.
Bake, reversing baking sheets once, until firm, about 15 minutes. Cool for 10 minutes.
Reduce the oven temperature to 300 °F (140 °C). Place the logs on a cutting board. Cut the logs with a serrated knife on the diagonal into 1/2" thick slices. Place the slices upright on the baking sheets, 1" apart. Bake until the cut sides feel dry to the tough, about 20-25 minutes. Cool completely on wire rack. Store in an airtight container.