Preheat the oven to 180C. Line a baking sheet with parchment.
In a small bowl, whisk together the amaranth flour, rice flour, almond meal, cocoa powder, baking soda, and salt. Add the chocolate chips and nuts and toss well to mix. Reserve.
In a large bowl, beat together the butter, eggs, sugar, and vanilla. Add the dry ingredients from the reserved bowl and bring together with your hands to make a firm dough.
Divide the dough in half. On a lightly oiled surface, pat one half together firmly and roll into a log. Flatten gently. Repeat with the other half.
Transfer the logs gently onto the baking sheet, evenly spaced apart, brush with the reserved egg white, and sprinkle with sugar.
Bake for 25 minutes, until the dough feels firm to the touch. {Keep a close eye as the oven is quite hot and the chocolate can burn}
Remove pan from the oven and cool 10 minutes. On a cutting board, use a serrated knife to diagonally cut the cookies into 1/2-inches slices.
Reduce heat to 150C. Lay the cookies cut side down on baking sheets and return to the oven for 35 minutes {or more if you like your biscotti crisper}, turning the baking sheet midway during baking, until the cookies feel mostly firm.