1. Preheat oven to 425°.
2. Lightly spoon flour into dry measuring cups; level with a knife. Combine flour, baking powder, and salt in a large bowl, stirring with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Stir in cheese, tomatoes, and basil. Add buttermilk and egg whites, stirring just until moist.
3. Turn dough out onto a lightly floured surface; knead lightly 4 times with floured hands. Pat dough into an 8-inch circle on a baking sheet coated with cooking spray. Cut dough into 8 wedges, cutting into but not through dough. Coat top of dough lightly with cooking spray. Bake at 425° for 15 minutes or until scones are golden.