Martha Stewart and Rachel Ray have recipes for hot buttered rum. Both recipes make 4 servings and use 6 ounces of rum, which is 3/4 of a cup.
For Martha Stewart's Hot Buttered Rum, beat 4 ounces of room temperature unsalted butter, 1/2 cup of packed dark brown sugar, 1 teaspoon of finely grated orange zest and 3/4 teaspoon of ground cinnamon. Also add in 3/4 teaspoon of ground ginger and 1/2 teaspoon of ground nutmeg. Blend in a mixer on medium speed for about one minute, until combined.
Combine 2 tablespoons of the spiced butter with 1 1/2 ounces of dark rum in 4 heatproof glasses. Pour 3/4 of a cup of boiling water over each, stir and top with a squeeze of fresh orange juice.
For Rachel Ray's Hot Buttered Rum, beat 2/3 cup of packed dark brown sugar, 1/2 cup of unsalted butter, 1/4 cup of honey and 1/2 teaspoon of ground cinnamon with an electric mixer. Also add in 1/4 teaspoon of ground nutmeg, an 1/8 teaspoon of ground cloves and a pinch of salt. Blend until smooth.
Transfer to a large measuring cup, then add 3/4 cup of rum and 2 cups of boiling water. Stir until the butter mixture dissolves. Divide into 4 mugs, and garnish each with a cinnamon stick.