Step 1

Heat the milk, bringing it almost to a boil. In another saucepan, melt the butter with the sugar, then incorporate the flour and the cocoa powder sifted together. Gradually pour the milk in and combine; this will prevent lumps from forming. Bring back over the heat and cook for 4-5 minutes after it comes to a boil.

Step 2

Turn off the heat, add the finely chopped chocolate, and stir until it has melted. Pour the mixture into a mold brushed with rum. Tap it on a work surface to prevent air bubbles from forming; cover with aluminum foil and let cool, then refrigerate for at least 4 hours. Briefly immerse the mold in hot water, then turn the custard over onto a serving dish.