Heat oven to 190C/fan 170C/gas 5. Butter two 20cm sandwich tins and line with non-stick baking paper. In a large bowl, beat all the cake ingredients together until you have a smooth, soft batter.
Divide the mixture between the tins, smooth the surface with a spatula or the back of a spoon. Once the cake mix is in the pan, poke into it thick slices from the 4 peeled and cored small eating apples (Cox’s are ideal). Then bake for about 30 mins (half way through baking add the streusel mix on top of one of the cakes, see next step below) until golden and the cake springs back when pressed. Turn onto a cooling rack and leave to cool completely.
Meanwhile, roughly rub together the light muscovado sugar, plain flour and butter. Mix in the roughly chopped blanched hazelnuts. Half-way through baking, sprinkle the streusel mix on top of one of the cakes.
To make the filling, beat the butter until smooth and creamy, then gradually beat in icing sugar. Beat in vanilla extract if you’re using it. Spread the butter cream over the bottom of the sponge that doesn’t have the streusel on top and sandwich the two halves of the cake. Keep in an airtight container and eat within 2 days.