Preheat the oven to 180 degrees centigrade. Grease and line a spring-form cake tin
Sift together the flour, custard powder or corn flour and bicarbonate and set aside
In another bowl, mix together the creme fraiche/yogurt, oat milk and vanilla and set aside
Cream together the margarine and sugar until light and fluffy
Alternately add the flour and milk mix (about 1/3rd of the mix each time) and stir well between each addition.
Pour into the cake tin and level off.
Peel and core the apples and finely slice
Decoratively lay the apple slices onto the top of the cake.
Mix together the Demerara sugar and cinnamon and sprinkle an even layer over the top of the apple slices.
Bake for 40-50 minutes until he cake is cooked through and golden on top.
Cool briefly in the tin before unmoulding and cooling on a wire rack. Once cool, dust with icing sugar and enjoy.