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In a medium bowl, beat the butter and sugar together until light and fluffy. Gradually add the eggs followed by the flour and instant coffee. Stir most of the walnuts (if using) into the batter, reserving a few for decoration. Transfer to a microwave-proof dish and cook in the microwave on full power for 2 mins. Reduce the power to medium and cook for 2 mins.
After 4 mins, check to see if the cake is cooked – it should be risen and spring back when touched. If it needs a bit longer, cook on medium for 1 min more at a time, checking after each minute until the cake is cooked. Remove and allow to cool.
Meanwhile, make the coffee butter cream. Dissolve the coffee in the milk, then add the butter and icing sugar. Beat until smooth, spread over the cake and top with the reserved walnuts.