Preheat oven to 350F. Line a cookie tray with baking paper.
Combine first 6 ingredients in a large bowl. Cut butter into dry ingredients using a pastry blender, or rub it in with your fingers (as if making pie dough or crumb topping). Set aside 2 tablespoons of chopped almonds; add the rest to flour mixture.
In a small bowl, whisk together egg, egg white, buttermilk and vanilla extract. Pour into the bowl with the flour mixture and stir together gently until the mixture comes together. You may need to use your hands to finish mixing it by lightly kneading the dough a few times.
Place dough on the lined cookie tray and shape into a log roughly 10″ long, 5″ wide, and then flatten it slightly to 1″ thick. Sprinkle reserved chopped almonds on top and bake for 30-35 minutes until golden brown. Let cool almost completely on a wire rack. Turn the oven down to 325F.
Slice loaf across the width using a sharp knife or serrated bread knife, into 3/4″ thick slices. Place the biscotti cut side down back onto the cookie tray. Bake again for 10 minutes, then flip them over and bake for another 10 minutes. Transfer to a wire rack and allow to dry out overnight.