Preheat the oven to 170°C/gas mark 4.
Grease and line a 20cm round baking tin with greaseproof paper.
For the topping, melt butter and stir in the muesli, mix well and reserve. In a frying pan heat 25g butter and 1tsbp sugar and the apple. Cook until the apples are soft and golden, and set aside.
Beat the remaining butter and sugar until pale and fluffy. Add the eggs gradually. Then carefully fold in the flour and baking powder, which have been sifted together.
Add creme fraiche and mix until smooth. Spoon about half the mixture into the greased cake tin. Add half the topping mix. Cover with the rest of the cake mixture. Place the cooked apple on top and cover with the rest of the topping.
Bake for about 45 minutes. If the cake looks like it is browning too much then loosely cover with foil or greaseproof paper to protect the cake.
The cake is cooked when a skewer inserted into the cake comes out clean. Cool in the tin for 10 minutes, before removing and serve when ready.
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