Grease 2 circular 8 inch cake tins and line the bottom with baking paper. Preheat the oven to 160C Fan.
In a large bowl, cream your butter and sugar together using an electric mixer until pale and light.
Crack eggs into a jug and gradually add one yolk at a time and thoroughly mix in between each addition. Beat in your vanilla extract and milk.
Gradually fold in your flour, baking powder and xantham gum. Make sure you do this a little at a time and don't over-fold the mixture.
Divide between 2 x 8 inch cake tins evenly and smooth the surface.
Bake for 45 minutes until golden and the sponge springs back. You can insert a skewer and if it comes out clean, its cooked. If not, put it back in the oven for a little longer.
Leave the sponges in their tins for about 5 minutes before turning them onto a cooling rack.
Whilst the sponges are cooling, make your buttercream. Cream the butter in a mixing bowl until pale and light. This can be done by hand or using an electric whisk.
Gradually add in your icing sugar and your vanilla extract/paste and mix. If you need to add 1-2 tbsp milk to loosen, you can do so at this stage.
Once the cakes are cooled, spread the jam on the bottom half of the cake, try not to put it close to the edges as it will ooze out!
Spread the buttercream underneath the top half and place on top so they sandwich together.
Dust a little icing sugar over the top. Enjoy!