To make this delicious coffee cake, pre-heat your oven to 160C, 325F or gas mark 3, and grease the sides and line the bottom of two 8-inch sandwich tins. Cream together your butter and sugar until light and fluffy. Add the eggs, one at a time, making sure each one is well beaten in before adding the next. Sieve in the flour and baking powder then gently fold them into the mixture. Next, dissolve the espresso powder in the hot water then fold this into the cake batter until it is well combine and your batter is lovely light brown colour. Divide the cake batter evenly, between your two cake tins and bake in the middle shelf of your oven for 20- 25 minutes until the sponge has risen and is springy to touch. Once the cake is ready leave to cool in the tins for a few minutes before turning out on a wire rack to completely cool down.
Whilst you are waiting for the cakes to cool you can make your buttercream icing. First up, beat your room temperature butter in a bowl until nice and soft. Then add a few tablespoons of icing sugar and mix until it is well combine. Keep adding the icing sugar and mixing thoroughly until it has all been used. Dissolve the espresso powder into the hot water then mix well into your butter and icing sugar mixture, and you will be left with a delicious light brown butter cream icing.
Once the cakes are completely cooled, place half of your buttercream on top of your bottom sponge and spread out evenly. Place the other sponge on top and spread over the reminding buttercream and sprinkle over bashed walnuts (optional).