Add the first 4 ingredients to a bowl and whisk well.
In another big bowl add the two flours, salt and baking powder. Using a pastry blender or two knives, cut in the butter until the mixture resembles coarse meal. Add the chocolate and gently fold it in.
Pour the wet ingredients over the dry ingredients and stir until moist.
Turn the dough over onto a floured surface and gently knead it a few times until the flour is incorporated. Place the dough onto a baking sheet and using a rolling pin roll the dough until it's about 3/4 of an inch in thickness.
Brush the dough with the egg white, then cut it into 12 pieces, you can use a pizza cutter or a knife. Sprinkle the top with sugar.