Place dough on a floured work surface and roll with a floured rolling pin to 3/4-inch thick. Dip a 2-inch cutter into flour and cut out scones as close to one another as possible. Place on prepared baking sheets, with space in between; let stand 10 minutes. Brush tops with egg, and bake until golden brown, 10 to 12 minutes. Remove to wire rack to cool. Serve warm with butter, clotted cream, fruit preserves, or jam. Makes about 1 1/2 dozen.
Baking powder scones: Omit baking soda and cream of tartar and substitute 4 tsp. baking powder in dry ingredients. Replace sour cream with 1 1/4 cups milk.
Cheddar-thyme scones: After combining butter and dry ingredients, stir 1 1/2 cups grated Cheddar cheese and 1 Tbsp. chopped fresh thyme into flour mixture before adding sour cream. Sprinkle tops of scones with an additional 1/2 cup grated Cheddar cheese before baking.
Raisin scones: Add 1/4 cup sugar to dry ingredients. After combining butter and flour mixture, stir in 1 cup raisins.