Prepare the streusel: chop the apple finely and melt the butter. Mix the brown sugar, oat flour, cinnamon, nutmeg, apple, and vanilla sugar in a bowl, then add in the butter. Stir until well combined then set to one side.

For the cake: pre-heat the oven to 175°C (350°F) and prepare a small springform pan (mine was 18cm/6 inch). In a processor, cream the butter and sugar together and then add in the egg and continue to mix just until the egg is well incorporated. Add the crème fraiche, mix, and then lastly, add the flour and vanilla sugar. Process until everything is nice and smooth. Lastly, add in the milk one tablespoon at a time.

Pour half of the batter into the prepared tin and sprinkle half of the streusel mixture over the top. Top with the remaining batter and then the remaining streusel.

Bake for around 40-45 minutes, until an inserted skewer comes out clean. Remove from the oven and allow to cool completely.

For the drizzle: combine the Greek yoghurt with as much icing sugar needed to make a runny, but thick, drizzle. Remove the sides of the springform and then drizzle icing over the cake. Cut into slices and enjoy!