Pre-heat your oven to 140degrees fan assisted (160 conventional / Gas Mark 3)
Cream together the butter and sugar.
Add the eggs one at a time and combine thoroughly - you can add a teaspoon of the flour in between each egg if your mix starts to split.
Fold in the two flours until the mixture is nicely combined.
Fold in the coffee mixture, chopped walnuts and chocolate chips until they are evenly dispersed through the mix.
Pour the mixture into a lined baking tin (we used an 8 x 10 inch) and spread so it's even across the tin.
Pop into the oven for about 25 minutes but you may need longer dependent on your oven - when you pop in a skewer, you want it to come out clean.
For the Buttercream
Cream the butter in a bowl until really soft and fluffy. Add the icing sugar and beat until completely incorporated (if you have a preferred method do use that. If not, we recommend adding a couple of large tablespoons of icing sugar at a time to reduce the mess!).
Once all the sugar and butter are combined, pour in the coffee mixture and beat till fully combined. If you feel it needs a little more loosening (and this will be to taste), you can add a teaspoon or two of milk.
Spread the coffee buttercream over the cooled sponge and sprinkle (or artfully place!) the remaining walnuts on top.