Melt butter in a medium size saucepan over low heat
Remove from heat and add the bottle of rum, stir to incorporate. Reserve the empty bottle.
Store the rum mixture in an airtight container with a wide mouth, and let rest for 24-48 hours, agitate occasionally to encourage the incorporation of the butter into the rum.
Place in the freezer for 2 days
Strain with very fine mesh or coffee filter back into the empty rum bottle