Heat your oven to 180°c/160°c Fan/ Gas 4 Grease and line two 20 or 22cm cake tins.
In a bowl, mix all the flours together with the baking powder and xanthan gum.
In another bowl, beat the butter and sugar until light and fluffy. Add the eggs, one at a time, beating well. Add the flour mixture and fold in using a large metal spoon or spatula.
Divide the mixture equally between the tins and gently smooth out with the back of a spoon. Bake in the oven for 25 minutes until golden and slightly shrunk from the sides. Leave in the tins for a few minutes, then remove and place on a wire rack to cool.
In a bowl, add the butter and sieved icing sugar and mix until smooth and creamy. Add the vanilla extract and mix again.
Place both sponges upside down and spread the buttercream on one, and jam on the other. Carefully sandwich them together. Dust with sifted icing sugar to finish.
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