In a large bowl, whisk the flour, protein powder, baking powder, optional spice, and salt. In a small bowl, whisk the egg, egg white, honey, melted butter, and vanilla until blended. Add the egg mixture to the flour mixture; add any stir-ins, if using. Mix until just blended. (it does not have to be completely cohesive at this point).
Transfer dough mixture to prepared cookie sheet. Using wet hands, press and shape dough to form into a 12×3-inch log.
Bake for 20 to 25 minutes, until the log is set and brown. Remove from oven and let cool for 15 to 20 minutes. Reduce oven temperature to 300F.
Slice biscotti at a 45-degree angle into 16 slices (about 3/4-inch thick). Lay flat on the same cookie sheet and return to the oven for 12 to 15 minutes until lightly browned. Flip cookies and bake for another 12 to 15 minutes.