Pre-heat the oven to 170 degrees Celsius (340 degrees Fahrenheit) and either grease your cake tin or line it with greaseproof paper. I prefer to grease it with butter as it means you have less waste at the end. If your tin is old and well used, though, paper may be the best way to go as it’ll have less chance of sticking.
Make a very small, very strong black coffee using two desert spoons of instant coffee granules and the smallest amount of hot water that you can to get it all to dissolve. Certainly don’t use more than half an espresso cup of water. The more water you use, the sloppier your cake will be, and it might not bake properly.
If mixing by hand, sift the flour, sugar and baking powder into a large bowl to make sure there are no lumps, then melt the butter and stir that, the eggs and the coffee into the dry ingredients. Mix well until it is a smooth paste and you can see no dry flour or sugar in the mixture. Beat hard, using a circular motion with the spoon that lifts the mixture up from the bottom of the bowl to introduce as much air into it as you can.
If using a mixer, instead sift the flour, sugar and baking powder into the mixing bowl, add the coffee, eggs and melted butter and them mix at high speed for a couple of minutes until you get a smooth, light mixture.
Spoon your mixture into your baking tin or individual cup cake cases and put into the oven. If baking a single cake, cook for around 30 minutes, then test by taking it out of the oven and gently pressing the top (use gloves as it will be hot). If it springs back and is an even brown colour, it is finished. If not, bake a little longer and try again. You shouldn’t need to cook for more than 35 minutes.
If baking cup cakes, cook for 15 minutes before testing in the same way, and then 5 minutes more if they are not cooked.
If icing your cake, allow it to cool and then combine as much icing sugar as you can with a third of a tea cup of very strong black coffee (again, two desert spoons of instant granules will be about right). Mix thoroughly to get rid of any lumps. Keep adding more icing sugar until you have a thick paste and then spread this across the top of the cold cake. Allow to harden and then serve.