Preheat the oven to 350°F (180°C)
Prepare your baking sheet with parchment paper
Whisk together flour, cocoa powder, baking powder, and salt in a bowl. Set aside.
Beat together brown sugar, white sugar, egg, butter, vanilla extract, and Cointreau with an electric or stand mixer until smooth.
Add in ⅓ of the dry ingredients, mix until incorporated, then repeat twice more. It should resemble a thicker cake batter.
If the dough dries, add in 1 tablespoon of water at a time until the dough thins to the correct consistency.
Fold in the toasted almonds and chocolate chips until just mixed. The dough should now resemble a thin cookie dough and be workable.
Turn out the mixing bowl onto a lightly floured surface. Dust your hands with flour, as the dough will be sticky.
Roll out the dough into two cylinders and place them on your prepared baking sheet. They should be the full length of the sheet. Gently flatten the tops.
Bake for 30-40 minutes, watching for the biscotti to spread out.
Remove from the oven, turning it down to 325°F (160°C), and let the cookies cool completely.
Using your serrated knife, slice them down into ½-inch pieces. Place the slices back on the cookie sheet, then brush with egg wash.
Bake for 15 minutes, or until firm and crispy.