Place vanilla wafers in a food processor and pulse until you have fine crumbs. If you don't have a food processor, place them in a zip-top bag and crush them with a rolling pin.

In a large bowl, whisk together the vanilla wafer crumbs, 1 cup of powdered sugar, finely chopped pecans, and cocoa powder until well combined.

In a separate smaller bowl, whisk together the dark rum and light corn syrup until the syrup has dissolved.

Pour the rum mixture into the dry ingredients. Mix with a spatula until everything is moistened and a sticky dough forms. Let the mixture sit for 20-30 minutes to allow the crumbs to absorb the liquid.

Place the remaining 1/4 cup of powdered sugar in a shallow dish. Scoop tablespoon-sized portions of the dough and roll them between your hands to form 1-inch balls. Roll each ball in the powdered sugar to coat.

Place the finished rum balls in an airtight container. For the best flavor, let them rest for at least 24 hours before serving. Store at room temperature for up to two weeks.