Preheat your oven to 180°C and grease and line 2 20cm round loose bottom cake tins.
Make the cake by whisking butter and caster sugar in a large bowl until creamy. Add the eggs one at a time and continue to whisk until combined. Add the vanilla extract and cooled coffee and whisk again.
Fold in the self-raising flour, xanthan gum and baking powder. Fold in the finely chopped walnuts.
Divide the mixture between the 2 lined baking tins and bake in the oven for around 30 minutes (checking after 25), until golden and a skewer comes out clean. Leave to cool completely.
Make the buttercream by beating butter with an electric whisk until soft. Gradually add icing sugar, continuing to beat for up to 5 minutes until combined and creamy. Add the cooled coffee and whisk to combine.
Sandwich the cakes together with a layer of buttercream. Spread a layer of buttercream on top of the cake and decorate with the walnut halves.