In food processor, combine flour, white sugar, baking powder, salt and the zest of the lemon. Pulse once or twice until combined.
Add in the butter, yogurt, vanilla, and egg white. Pulse until dough is just combined and looks like sand. Dump the dough out unto floured surfaced and press together into a large circular disk. Roll out to ½ inch thick. Either cut with a floured cookie cutter or a knife into triangles.
Place on cookie sheet.
Mix egg yolk with 1 tablespoons water and brush over each scone.
Bake for 12-15 minutes, or until browned.
While the scones are baking, prepare glaze by mixing together the juice of a lemon and the powdered sugar.
Once scones are cool to the touch, dip each scone into the glaze and let sit until glaze sets.