Preheat oven to 400 degrees F.
Cut butter into little pieces in large bowl.
With pastry blender, mix flour, sugar, and baking powder into butter.
Using fork, quickly and gently stir in milk and yogurt just until dough clings together.
Turn dough onto a floured surface (dough will be STICKY!).
Gently knead in currants, taking care not to overwork dough.
Halve dough and pat each half into a 6-inch circle.
Cut each circle into 8 wedges.
Place wegdes on a floured baking sheet, making sure they do not touch.
In a small bowl, beat together the egg yolk and water.
Brush mixture over wedges.
Bake in 400 F oven for 18 to 20 minutes, or until the tops are golden.
