Gather your ingredients and equipment: a small saucepan, stirring spoon, fine strainer, and a mug.
Place the small saucepan over low heat and add the butter. Melt it gently until it starts to shimmer and smells rich, about 2 minutes.
Add the ground cinnamon, nutmeg, and a pinch of clove to the melted butter. Stir for about 30 seconds until fragrant, watching for a warm, aromatic aroma to fill the air.
Pour in the dark rum and sprinkle in the brown sugar. Stir gently until the sugar dissolves completely and the mixture feels warm and smooth, about 1 minute.
Gradually add the hot water or apple cider to the saucepan, stirring continuously until everything is well combined and heated through.
Taste the drink and adjust by adding more rum for a stronger kick or more sugar for sweetness, if desired.
Place the fine strainer over your mug and pour the hot buttered rum mixture through it to remove any spice solids for a smooth drink.
Serve immediately, optionally topping with a dollop of whipped cream or a sprinkle of cinnamon for extra coziness.