Melt the butter in a saucepan over medium heat. Slowly stir the brown sugar into the butter, allowing it to melt completely. Bring to a boil and cook until the sugar caramelizes and gives off a burnt smell, about 2 minutes. Remove from heat and pour the hot water into the saucepan; mix until combined. You will have a sauce that is very liquid. Pour over pieces of cake and serve immediately. Best for stale white cake.