1. Preheat the oven to 180°C/Gas Mark 4. Grease a 23cm (9in) springform tin and line with baking paper. 2. Bring the water to a boil in a saucepan. 3. Turn the heat down to low, then add the chocolate chunks and coffee. 4. Stir continuously until all the chocolate is melted, then add the butter and stir until combined. 5. Whisk the egg yolks with the sugar until fluffy and doubled in size. 6. In a separate bowl, whisk the egg whites until stiff peaks form. 7. Fold the melted chocolate mix into the egg yolk mix. 8. Carefully fold one-third of the egg whites into the mix. 9. Now fold the rest of the egg whites in, being careful not to overwork it. 10. Pour the mix into the prepared tin. Bake for 50 minutes, or until a skewer comes out clean. 11. Leave the cake to cool on a rack before transferring into the fridge for a few hours. 12. Serve your flourless chocolate cake with your preferred toppings, or on its own.