Preheat the oven to 180°C/350°F/Gas Mark 4 Grease and line the base of the cake tin with baking parchment. In a large bowl, whisk the eggs and caster sugar together, until creamy and pale in colour.
Put the chocolate into a heatproof bowl (600ml/1 pint) and place over a saucepan of simmering water. Melt the chocolate and add the salted butter. Stir to combine. Quickly blend the Rombouts coffee into the chocolate mixture. Remove from the heat and cool slightly. Stir in the chocolate mixture into the egg and sugar mixture. Add the flour and beat all the ingredients together until the texture is smooth.
Spoon the cake mixture into the cake tin, place on the middle shelf of the oven and bake for 35 minutes.