1/2cup(1 stick) cold butter, cut into 1 inch pieces
1egg white, beaten
Preheat oven to 375°F.
In a large bowl, add flour, baking powder, baking soda, salt and sugar. Cut in butter into dry ingredients until it's pea size crumbles.
In separate bowl, stir yogurt, egg and vanilla together. Stir into flour mixture. Pour onto a lightly floured surface and knead five or six times. Divide dough in half. Pat each half into about an 8 inch circle. Carefully place one dough circle onto a greased or lined baking sheet.
In a small bowl, combine streusel ingredients until crumbly. Sprinkle over bottom dough circle. Place remaining dough circle over streusel filling. Pinch edges together gently. Using a sharp knife, carefully cut into 8 wedges. Place on baking sheet.
Brush with egg white and sprinkle with sugar.
Bake for 20-25 minutes or until golden brown.
For smaller sized scones, divide dough into four portions, make two 4-inch circles, add streusel to each, top with two more circles and cut each into 8 scones, making 16 mini scones.