Preheat oven to 350 degrees. Combine flours with sugar and baking powder. Grate the butter and quickly blend it in with your fingertips. (The texture should be grtty.) Stir in the egg, followed by a cup of buttermilk. Continue to add milk a few spoonfuls at a time, until batter has the consistency of biscuit dough--thick and just moist enough to adhere to itself.
With floured hands, pinch off golf-sized pieces of dough. Roll into balls and place opn a greased baking sheet. Bake for 20 minutes, or until a cake tester inserted into the dough comes out clean. The surfaces should be golden brown.
Let oven cool to 200 degrees. Pile the cooked pieces into baking dishes or into cookie sheets. Leave in the oven overnight to dry. The next morning let the rusks cool and then store in an airtight container.