I started making hot buttered rum from a Trader Vic recipe 15 years ago, and I have gradually evolved it by adding more spices, more butter, and less rum.
The way you make a hot buttered rum is to add some “batter” to some rum, and heat it. I like to keep a made-up batch of the batter in the refrigerator in an old margarine tub. For parties I make up a fresh batch.