Melt chocolate over a double boiler, stirring until smooth.
Remove from the heat and whisk in 1/2 cup of sugar and the corn syrup, then the rum. In a separate bowl, stir together the vanilla wafer crumbs and walnuts.
Add the chocolate mixture to the nut mixture and stir until well blended.
Place the remaining 1/2 cup of sugar in a shallow bowl.
Roll the dough into 1 inch balls and roll the balls in the sugar to coat evenly. Cover and refrigerate at least overnight for best flavor. These can be refrigerated up to 5 days.