Chop the chocolate and melt it with the butter over low heat. Once melted and homogenized, mix with the cocoa and coffee. Allow the mixture to cool and in the meantime, beat the eggs with the sugar and flour well.
Pour the egg mixture in to the chocolate one and stir until you get uniform consistency. Pour the mixture into a baking form with removable walls, which you've covered with greased baking paper.
Bake this cake in a preheated 356°F (180 °C) oven for about 15 minutes. The cake will remain slightly runny on the inside and have a nice crust on the outside.