Place butter into a heavy bottomed saucepan over a low heat.  Once melted, increase the heat slightly to a gentle simmer.  It will foam and gurgle as the water in the butter evaporates.  Simmer uncovered for 20 minutes, or until the milk solids start to brown on the bottom of the pan.

Remove from heat and allow to cool.  Once cooled, skim the froth from the top and pour into a clean glass jar.  Discard the solids on the bottom of the pan.  If you like, you can strain through muslin cloth.