In a heatproof bowl over a saucepan of simmering water (double boiler), melt the dark chocolate and butter together. Stir until smooth and glossy. Remove from heat and let it cool slightly.
In a large bowl, mix the powdered sugar, crushed vanilla wafers (or graham crackers), cocoa powder (if using), and a pinch of salt.
Pour the slightly cooled chocolate-butter mixture and the rum into the dry ingredients. Mix well until the dough comes together. It should be thick, sticky, and easy to roll.
Chill the dough for about 30–60 minutes. Once firm, scoop out small portions and roll into balls. Roll each ball in your preferred coating: chocolate sprinkles, cocoa powder, or shredded coconut.