Preheat the oven to 350°F/176°C. Line a baking sheet 'with parchment paper, and spread the walnuts in a single layer. Toast them, stirring occasionally, until they're lightly golden and fragrant, about 5 minutes. Remove from the heat and cool the sheet on a wire rack, keeping the oven on.

In the bowl of an electric mixer fitted with the paddle attachment, combine the flour, cocoa powder, brown sugar, honey, butter, baking powder, and salt. Beat on low speed until the ingredients are well combined, about 4 minutes.

In a medium bowl, whisk together the eggs, egg yolk, orange juice, orange zest, and vanilla. Add the mixture to the flour mixture and beat on low speed to combine. Add the cooled nuts, and beat just to combine.

Turn the dough out onto a generously floured surface and divide it in half. Shape each portion into a 16"x2" log, and transfer to a parchment-lined baking sheet, spacing them about 4 inches apart.

Bakק until the logs are firm to the touch, about 20 minutes. Transfer the logs still on the parchment paper, to a wire rack and let cool for about 5 minutes.

Reduce the oven temp to 250°F/120°C. Transfer the warm logs to a cutting board, and using a serrated knife, cut the logs on the diagonal into 1 1/2-inch thick slices. Arrange the slices, cut side up, on the baking sheet.

Bake until the biscotti are dry and firm to the touch, about 1 hour. Remove the baking sheet from the oven and let the sheet of biscotti cool completely on the rack.