Anna's Light and Tender Baking Powder Biscuits

This is my Grandma Anna's recipe for biscuits. She always says that the key is to handle the dough very little and they will turn out flaky. I still can't get them as good as hers, but heres the recipe.

3 c flour
4 1/2 t baking powder
2 T sugar
1/2 t salt
3/4 t cream of tartar
1 egg, beaten
1 c milk
3/4 c butter

Preheat oven to 425. Combine first 5 ingredients. Cut in butter until crumbly. Add the egg and milk and stir until just mixed. Turn out on lightly floured surface. Roll out to about 1" and cut with a biscuit cutter or cut into 2" squares with a pizza cutter. Bake for 12-15 minutes.