Time: 1h 20 minutes
Preheat the oven to 180C/Gas 6
Line 2, round 20cm sandwich tins with baking parchment. Grease the sides and dust with a little flour.
If using instant coffee, mix it with 125ml of boiling water and stir till dissolved. Divide your coffee in half and sweeten one half with the caster sugar.
Beat together the butter and muscovado sugar with an electric whisk until smooth and light. Beat the eggs in a small bowl and add them to the cake mix a little at a time, beating between each addition. Don’t worry if the mix curdles slightly-this will right itself once the flour is added.
Add ½ of the unsweetened coffee to the mix and then add the flour, baking powder and milk. Beat well again and finally stir in the walnuts.
Spoon the mixture into the two cake tins, dividing it equally. Smooth it over and bake in the oven for 20-25 minutes until the cake has left the sides of the tin and is firm to the touch. Turn out onto a cooling rack and leave to cool completely.
To make the icing-mix the muscovado sugar with the remaining unsweetened coffee until it forms a thick paste. Melt the butter in a small pan or microwave, and then beat together with the sugar paste and cream until it is light and fluffy and has thickened enough to spread easily.
When the cake is cooled, drizzle the sweetened coffee over both halves of the underside of each cake. Spread 2/3 of the icing over one half of the cake and place the other half on top.
Spread the rest of the icing over the top of the cake and decorate with the reserved walnut pieces. Dust with a little cocoa powder.