Preheat oven to 350F and line 2 baking sheets with parchment paper.
In a large bowl, whisk together eggs; sugar; vegetable oil and orange zest.
Sift the flour, baking powder and pinch of salt. Gradually add the dry ingredients to the egg mixture with a wooden spoon until fully incorporated. The mixture will resemble a very soft bread dough.
Scrape out the dough onto a lightly floured surface and divide it into into three pieces.
If you are making all 3 variations, see *note below.
Knead in the ingredients of your choice into the dough until evenly dispersed. The dough will be sticky. Avoid adding too much extra flour and use a bench scraper and/or damp hands to help you.
Roll each piece into a log, transfer onto the prepared baking sheet and flatten into a 30 cm by 8 cm rectangle.
Bake for approximately 25-30 minutes or until golden brown.
Transfer to a wire rack and let cool for 15-20 minutes. Reduce the oven temperature to 275F.
Place each log on a cutting board. With a serrated knife, slice each log into 1 cm biscotti. Transfer the biscotti back onto the baking sheets and continue baking for 30 minutes, flipping halfway.
Transfer to a wire rack and let cool before serving.