Preheat oven to 350°F (177°C). Line two large baking sheets with parchment paper or silicone baking mats. (Always recommended for cookies.) Set aside.

Whisk the flour, brown sugar, baking powder, cinnamon, salt, and orange zest together in a large bowl until combined. Using a pastry cutter or 2 forks, cut in the butter until the mixture is crumbly. Stir in the walnuts. Set aside. In a medium bowl, whisk the eggs, orange juice, oil, and vanilla extract together. Pour into the flour/butter mixture and gently mix together until everything is just barely moistened.

Turn dough out onto a lightly floured surface and with floured hands, knead lightly until the dough is soft and slightly sticky, about 8-10 times. If it’s uncontrollably sticky, knead 1-2 more Tablespoon(s) of flour into the dough. Divide the dough in two and place each half onto a baking sheet. Shape each half into an 8×4-inch long rectangle, patting down until each is about 1/2 inch thick. Using a pastry brush, lightly brush the top and sides of each biscotti slab with egg wash.

Bake for 25-26 minutes, or until the top and sides of the biscotti slabs are lightly browned. Remove from the oven, but do not turn off the heat. Allow to cool for 10 minutes. Once the slabs are cool enough to handle, cut each into 1 inch thick slices. Set slices cut sides upright, 1/4 inch apart, on the baking sheets. Return to the oven to continue baking for 8 minutes. Turn biscotti over and bake other side for 8 minutes. The cookies will be slightly soft in the centers with harder edges. Remove from the oven and allow to cool for 5 minutes on the baking sheet. Transfer to a wire rack to cool completely before dipping in chocolate. The biscotti becomes crunchy as it cools. Save the baking sheets for the next step.

Melt the chopped chocolate in a double boiler or (carefully!) use the microwave. For the microwave, place the chocolate in a medium heat-proof bowl. Melt in 15 second increments, stirring after each increment until completely melted and smooth. Dip each biscotti cookie in the melted chocolate and place back onto the baking sheets. Sprinkle with any additional orange zest if desired. Allow chocolate to set in the refrigerator or at room temperature.

Biscotti will stay fresh covered at room temperature or in the refrigerator for 1-2 weeks.