(To make sour milk, place 1 Tbsp lemon juice in a measuring cup. Add milk to 1 ¼ cup mark. Let stand 5 minutes)
Sift together flour, baking powder and cream of tartar, working out all the lumps. Stir in sugar and salt and mix well. Using a fork or pastry blender, work in softened butter till mixture is crumbly. Make a well in the center of the mixture. Add buttermilk or sour milk. Stir with a fork till just moistened.
Knead the dough on a lightly floured surface, folding it and pressing it 4 or 5 times till it sticks together. Pat or roll it ¾ inch thick. Cut into circles or shapes with biscuit cutter or cookie cutter. Place biscuits 1 inch apart on an ungreased baking sheet. Bake for 10-14 minutes until golden brown.