In a medium bowl, combine the 2 types of chocolate and melt over a double boiler. (Or in a metal bowl over a pot of simmering water.) Whisk in the butter pieces. Whisk in the sugar and the rum.
Put the sprinkles in a bowl. Roll the chocolate mixture into small balls, about 1 1/2 inches each. Roll them in the sprinkles until they are fully coated. Store them in an airtight container.
Recipe courtesy of Alex Guarnaschelli