Soak the oat flakes in boiling water for 5 minutes.
Add the melted butter, yogurt and beaten eggs, then mix until smooth.
Stir in the sugar and vanilla sugar.
Dice the apple and add it to the batter with the raisins or frozen berries.
Fold in the baking powder until evenly distributed.
Grease or line a baking tin and pour in the batter, smoothing the top.
Sprinkle chopped walnuts evenly over the surface.
Bake at 180°C / 350°F for 40–50 minutes, until golden and a skewer comes out clean.
Cool in the tin for 10–15 minutes, then transfer to a rack and slice once warm or cooled.