Preheat the oven to 350°F (177°C). Line a large baking sheet or two smaller ones with parchment paper.
Whisk flour, salt and baking powder in a bowl. Set aside.
Add in butter and sugar in a mixing bowl and beat on medium for 3-4 minutes, scraping the sides halfway through. The mixture should be light and fluffy. Add in orange zest and mix.
Add in eggs one at a time and mix until combined. Add in vanilla extract and mix.
Add the flour mixture and mix on low speed until almost combined. Don't overmix, a few traces of flour is fine. Fold in the chopped pecans.
Divide the dough in half, place each half on a baking sheet and shape them into two 3x5 rectangles (1-inch thick). Brush the tops with egg wash.
Bake in the preheated oven for 25 minutes or until it's lightly browned. Remove from the oven and let them cool down for 20 minutes. Reduce the temperature to 325°F (162°C). Cut each piece into 1-inch slices.
Place them cut-side down on the baking sheet. Return to the oven and bake for 20 minutes, turning them over half way through. Let them cool down for 5 minutes, then transfer to a wire rack to cool down completely.
Place chopped chocolate in a heat resistant bowl and melt it on low heat in a double boiler. You can use a microwave. Microwave for 10 seconds, take out the bowl and whisk, then do 5 seconds intervals and whisk in between. Continue until there is one or two bits of chocolate left, at this stage do not microwave again and just continue whisking until they're melted. This way you won't burn the chocolates.
Dip each biscotti in melted chocolate, let the excess drip and then place on a baking sheet. You can sprinkle it with chopped pecans or some orange zest. Allow the chocolate to set and enjoy! You can store biscotti in an airtight container for 1-2 weeks.