Crush the vanilla wafers into fine crumbs using a food processor or by placing them in a zip-lock bag and crushing with a rolling pin.

In a large bowl, combine the crushed vanilla wafers, 1 cup powdered sugar, cocoa powder, and chopped pecans. Stir until well mixed.

Add the corn syrup and rum to the dry ingredients. Mix with a wooden spoon or your hands until a thick, moldable dough forms. If the mixture is too dry, add a splash more rum or water.

Using a tablespoon or small cookie scoop, portion the dough and roll each piece into a 1-inch ball using your hands.

Roll each ball in the extra powdered sugar until evenly coated. Place on a parchment-lined tray.

Refrigerate the rum balls for at least 1 hour before serving to allow them to firm up. Re-roll in powdered sugar right before serving for the best look.