Preheat oven to 160ºC. Grease the base and sides of a 20cm springform cake tin and line it with baking paper.
Chop chocolate and butter into rough cubes. Place both in a small, heavy bottomed saucepan on low heat. Allow to melt, stirring occasionally until smooth and glossy. Remove from heat when completely melted. Add the coffee and stir well. Set aside to cool slightly.
Separate the egg yolks and whites in two large bowls. Whisk the egg whites for 1 to 2 minutes, or until soft peaks form.
Add the sugar to the yolks and beat for 4-5 minutes until pale and frothy. Add the cacao powder and beat for another minute until combined.
Using a silicone spoon, stir the melted chocolate into the yolk mixture. Gently fold through the egg whites until smooth.
Pour the batter into the prepared tin, smooth the top and place in the hot oven for approximately 1 hour, or until an inserted skewer comes out clean.
Allow to cool completely on a wire cooling rack, then carefully remove from the springform tin and slide onto a serving plate. Cut into slices and serve with ice cream or fresh cream and berries.