Put the beans and the liquid from the can in a saucepan and heat on the highest level. Add the butter and stir until melted. Reduce the heat and then mix a roux with water.
I always take half a cup of water and half a tablespoon of flour and stir it with a fork until there are no more lumps to be seen. Then pour the roux over the beans and season with salt and pepper to taste. Then stir again until the beans are creamy.