Heat your oven to 160C Fan / 180C and prepare your two 20cm (8 inch) circular tins.
Cream the butter and light brown sugar together until light and fluffy. I do this with an electric hand mixer.
Gradually add your four eggs one at a time, mixing inbetween each addition. Make sure you do this very gradually to reduce the potential of curdling (although it doesn't matter if it does).
Add your coffee, and mix it in (this needs to be 2 tbsp coffee dissolved in 2 tbsp of boiling water as per the ingredients above).
Fold in your gluten free flour, xanthan gum and baking powder until combined. Then fold in your finely chopped walnuts.
Divide your mixture between your two prepared cake tins. Bake in the oven for 25 minutes on the middle shelf, they should be well risen by the end!
Remove from the oven, briefly leave to cool in the tins before transferring to a cooling rack to cool completely.
Now to make the buttercream. Place your butter in a stand mixer (or electric hand whisk if you don't have a stand mixer), mix on its own on a high speed for about 5 minutes. The butter should change from a more yellow colour to being a lot more pale.
Add your icing sugar in 2-3 stages. Mixing for about 3 minutes between each addition.
Then add your coffee (again 2 tbsp of coffee dissolved in 2 tbsp of boiling water, left to cool) and mix it in so it's evenly through the icing.
Evenly top one sponge with plenty of your coffee butter cream, place your other sponge on top. (put enough butter cream in so it oozes a little out of the edges!)
Spread the rest of the coffee butter cream on the top of the cake. Add some walnut halves on top to finish.
Enjoy!