Mix by hand. Cream butter with sugar then add egg yolks and finally the rum. Soften the chocolate in a double boiler (don't let the inner pan touch the water in the outer pan) or a microwave. Combine the melted chocolate with butter mixture and beat with a spoon for about 5 minutes.
Form into balls using a plastic bag as a mitt. Roll the balls in chopped nuts or chocolate sprinkles, powdered sugar, or anything else.