Preheat oven to 325°F/160°C degrees. Grease an 8-inch springform pan. Wrap the bottom and sides of the pan with one large piece of foil, then repeat with another piece. This will ensure that water from the bath doesn’t seep into the cake. Bring a kettle of water to a boil.
Put chocolate, butter, water and instant coffee in a large heat-proof bowl and microwave in 20-second increments, stirring between each, until melted.
In a mixer bowl fitted with a whisk attachment, whisk eggs and sugar at high speed until light and fluffy and doubled in volume, about 5 minutes.
Using a spatula, fold half of egg foam into chocolate mixture (make sure the chocolate isn't too hot) until mixed. Fold in remaining foam until combined.
Pour batter into prepared cake pan and smooth the top. Place the pan inside a large roasting pan, and place in the oven, then pour enough boiling water from the kettle into the roasting pan to come halfway up the sides of the cake pan. Be quick so that the oven temperature won't drop quickly. Bake for 25 minutes until the cake has risen slightly and the edges are just beginning to set. The center will look undone but it will set once the cake has cooled (similar to cheesecakes). Carefully remove the pan from the water and transfer to a wire rack. Let the cake cool to room temperature. Cover and refrigerate for at least 6 hours or overnight until completely set. Keep cake in the fridge for up to 5 days.