In a small saucepan combine rum and banana liqueur. Place over medium-low heat and simmer gently for 2 - 3 minutes to reduce slightly and cook off most of the alcohol. (Be EXTRA CAREFUL if you are using a gas stove as the alcohol can flame up.)

Add butter and brown sugar, stirring until melted and fully combined.

Stir in pureed banana. Cook, stirring occasionally, until sauce thickens and becomes smooth and glossy, about 4 - 5 minutes.

Stir in the nutmeg and vanilla extract just before serving.

Keep warm for service or let cool and store in an airtight container in the refrigerator. Reheat gently before serving.