How to Make It
In a medium stainless steel saucepan, cook the butter over moderate heat until it turns medium brown and smells nutty, about 5 minutes. Remove from the heat. Very gradually pour in the rum; it will froth up. Let cool to room temperature, then transfer to a bowl, cover with plastic wrap and refrigerate for two days.
Skim off the solidified butter on the top. Strain the rum through a coffee filter.