Preheat your oven to 160C fan. Prepare a rectangular baking tin - mine is approx 32cm x 21.5cm. I grease the tin and then use baking paper - the baking paper should go over the edges so you can lift out the cake once it's cooled.
Dissolve your 3-4 tsp coffee in 1 1/2 tbsp of boiling water and leave to cool.
In a large mixing bowl, cream together your light brown sugar and your softened butter. Cream until lighter and fluffy. I use an electric hand whisk or a stand mixer.
Gradually add your eggs - 1 or 2 at a time, mixing in between each. (Remember if it curdles it doesn't matter really)
Fold in the gluten free self raising flour and xanthan gum to the mixture (this is a big mixture so make sure you carefully fold all the flour in properly).
Pour in your cooled coffee and fold through.
Add your finely chopped walnuts and carefully fold through once more.
Spoon your cake mixture into your pre-prepared baking tin and place in your preheated oven for 45 minutes until cooked through.
Leave to cool in the tin and then carefully lift onto a cooling rack.
To make your buttercream, start by dissolving your 2 tsp of coffee in 1 tbsp of boiling water and leave to cool.
Then ensure your butter is soft. I find it take a good hour out of the fridge, but never let it go too soft! Place your butter into a stand mixer and mix on a medium speed for about 5 minutes until the butter has gone a lot more pale in colour.
Add your icing sugar gradually. I add it in 2-3 stages and beat for about 3 minutes between each addition. Start your mixer slowly to save your kitchen from a real icing sugar mess, but then increase the speed to medium / high for each of your 3 minutes.
Next gradually add your cold coffee. Beat for 2-3 more minutes so it's fully combined, light and fluffy.
Spread the coffee buttercream all over the top of your cake.
Cut the cake into squares (you can cut the edges off first if you want, but it seems a waste!). Then finish by decorating with walnut halves / pieces. Enjoy!