Combine first 4 ingredients. Cut in butter with a pastry blender until mixture is crumbly.
Whisk together cream, egg, and vanilla; add to flour mixture, stirring just until dry ingredients are moistened.
Turn dough out onto a lightly floured surface. Pat dough to 1/2-inch thickness; cut with a 2 1/2-inch round cutter, and place on baking sheets.
Whisk together egg white and 1 teaspoon water; brush mixture over tops of scones. Sprinkle scones with additional sugar.
Bake at 425° for 13 to 15 minutes or until lightly browned.