To prepare filling, combine first 5 ingredients in a bowl. Cover and chill 2 hours.
Preheat oven to 400°.
To prepare shortcakes, lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 1/2 cup sugar, baking powder, and salt in a large bowl; stir well with a whisk. Cut in butter with a pastry blender or 2 knives until mixture resembles coarse meal. Add buttermilk; stir just until moist.
Turn dough out onto a lightly floured surface; knead lightly 4 to 5 times. Pat dough into a 9-inch round cake pan coated with cooking spray. Cut dough into 10 wedges, cutting into but not through dough. Combine water and egg white; brush mixture over dough. Sprinkle 2 teaspoons sugar over dough.
Bake at 400° for 25 minutes or until golden. Cool in pan 5 minutes on a wire rack; remove from pan. Cool completely on wire rack. Cut shortcake into 10 wedges. Split each wedge in half horizontally. Fill each wedge with 1/2 cup filling.