To make the hot buttered rum mix, place the butter, dark brown sugar, powdered sugar, cinnamon, and nutmeg in a bowl and use an electric mixer to blend until smooth.
To assemble each drink, place a heaping 1 1/2 tablespoons of the butter mixture into a mug. Add the rum. Fill the mug with boiling water and stir to combine.
Recipe Tips
Hot buttered rum mix will last indefinitely in the freezer.