Cut the butter and the dark chocolate bar into pieces. Put it all in a heavy-based saucepan. Melt the mixture over a low heat, stirring regularly.
Once the mixture is melted and smooth, take it off the heat.
The sauce is very runny and dark in colour. For a less intense chocolate taste (for children for example), remove a row from the chocolate bar before melting it with the butter.
Open the carton of sweetened condensed milk and pour all of it into the saucepan.
Whisk vigorously to thicken the mixture until, though the centre of the pan, it becomes a light-brown creamy mixture.
Continue to whisk until the mixture is completely smooth and very creamy.
The sauce will keep for several days. It will solidify in the refrigerator. Therefore, if you are using it on crepes, consider taking it out of the fridge 10 minutes before use.