In your food processor, combine the biscuits and sugar and blend until they’re finely ground, resembling a beige dust. Transfer to a large bowl. Mix in coconut.
In a saucepan, set over medium heat, melt the butter. Remove from heat and whisk in the chocolate powder. When dissolved, whisk in rum and milk.
Pour the chocolate mixture over the crumbs. Stir until it is completely incorporated.
Try to make a rum ball. If the mixture is too crumbly and you’re having trouble forming a ball add another tablespoon of milk. If the mixture is good, cover it tightly with plastic wrap or foil and chill the mixture in the refrigerator for 30 minutes.
Once chilled, roll out 50 balls. If desired roll each one in more coconut or chocolate powder. Store rum balls in an airtight contianer in the refrigerator for up to 1 week. They taste best served at room temperature (which softens the butter).