Mix the butter, sugar and vanilla sugar until frothy. Gradually stir in the eggs well. Mix the flour, baking powder and oat flakes and briefly stir in together with the yoghurt. Finally fold in the currants.
Grease the muffin tin or line it with paper liners, fill 2/3 with batter and bake at 180 degrees for about 20-25 minutes.
Dust them with a little icing sugar when they have cooled down.