Heat oven to 160C/140C fan/gas 3. Line and grease 2 x 18cm sandwich tins. Add the sugar and butter to a bowl and whisk until very fluffy and pale.
Whisk the eggs in a mug with a fork, then add them gradually to the mixture with 1 tbsp of flour each time. (Make sure you don't use all the flour.) When the eggs have been fully combined into the mix, add the rest of the flour and the baking powder and fold it in gently.
Add the dissolved coffee to the mixture, still folding. Divide into the sandwich tins and cook for 25-30 min until risen and firm and a skewer inserted into the middle comes out clean. Leave to cool in the tin for 5 mins then turn out onto a wire rack to cool completely. Can be frozen at this stage.
Meanwhile make the icing by beating the icing sugar with the butter until light and fluffy, then add the dissolved coffee. Whisk then cover and put to one side until ready to ice the cake.
Once the sponges have completely cooled, spread half the icing on the bottom of one (leaving around half for the top) and spread the strawberry jam on the bottom of the other, if using. Sandwich together, spread the remaining icing on top. Decorate with walnuts, or cherries if you prefer.