Preheat oven to 350 degrees. Position oven racks in the upper and lower thirds of the oven. Line two large baking sheets with parchment paper.
Place almonds in a single layer on a baking sheet and toast in the 350-degree oven for 10 minutes, or until light golden and aromatic. Remove and set aside.
In a large bowl, hand mix toasted almonds, sugars, cinnamon, baking powder and flour.
In a small bowl, whisk eggs. Add the vanilla and orange zest and whisk until well-blended. Add to the flour mixture.
Work the batter together with lightly floured hands. The mixture will be sticky, but persevere. Keep squeezing the batter with your hands until a dough starts to form. Once the dough is firm, form it into a ball. Divide the ball into four equal pieces.
Place one piece of dough on a lightly floured surface. Using your hands, roll it into a log that is approximately 8 inches long, 2 inches wide and 3/4 of an inch high. Repeat with remaining three pieces of dough. Place two logs on each baking sheet. Brush loaves all over with egg wash.
Bake for 40 minutes, rotating pans midway through. The tops of the loaves will be shiny and deep golden. Cool on a rack for about 20 minutes.
Place a loaf on a cutting board, and using a large serrated knife, slice cookies 3/4 of an inch thick on the diagonal. If the cookie crumbles, let it cool for a few more minutes. Don't let it rest too long, however, or it could become too hard to slice.
Place slices on their sides back on the baking sheets. Place in the still-warm oven with the temperature off and the door closed for 30 to 60 minutes. The longer they stay in the oven, the harder they will become. Remove from oven and cool completely before storing in an air-tight container, preferably a metal tin, which helps keep them crisp. Stored properly, biscotti will last up to a month.