Chop the chocolate and put it in a double boiler to melt. While this is happening, whip the butter and cacao powder to a fluffy consistency, adding the rum a little bit at a time. When the chocolate is melted, take it off of the heat, and stir the nuts into the chocolate. Fold the chocolate and nuts into the whipped butter mixture. Knead in about half of the powdered sugar, adding more as needed to achieve a dough-y consistency that will hold its shape.
Let the dough rest for about 30 minutes in the refrigerator. This is a great time to make yourself (and your best friends) a nice rum old fashioned, before you get to work. Then get comfortable around your kitchen table and enjoy the process of rolling out a season’s worth of rum balls. Finish by rolling the shaped dough in chocolate sprinkles.