Mix flour, sugar, baking powder, baking soda and salt together in bowl. Cut in margarine. In another bowl, beat eggs slightly, add sour cream and, using a fork, fluff together. Add to flour mixture (if dough is too soft, add more flour). (If you want to use raisins or shredded cheese, add them to the dry mixture first.) Pat (don't roll) dough to 1 1/2 inch thickness on floured cutting board. Using the top of a glass or mug, lightly floured, cut out buns and place on greased baking sheet. Brush tops with leftover egg white, lightly beaten. Bake at 450F for 10-12 minutes until browned.