In the bowl of a food processor, add the nilla wafer cookies. Process for about 10 to 15 seconds, breaking up any large pieces until fine crumbs form. Transfer them to a large bowl.
Place the pecans in the bowl of the food processor. Pulse 8 to 10 times until the pecans are finely chopped. Transfer to the large bowl with the nilla wafer crumbs.
To the cookie mixture, sift the sugar, cocoa powder, and salt over top.
In a small bowl, whisk together the rum and corn syrup until combined. Add the rum mixture to the dry ingredients and mix together with a spatula until the mixture is moistened evenly and holds together when squeezed. If there's any dry pocket, just pour a splash of rum in and mix.
Use a tablespoon to scoop out enough of the mixture to make a ball that's just over an inch in diameter. Roll in the palms of your clean hands to make a smooth ball. Repeat with the remaining mixture.
Roll the rum balls in more powdered sugar, cocoa powder, wafer crumbs, or finely chopped pecans to coat. Rum balls can be stored in an airtight container for up to 2 weeks or frozen for up to 3 months. They taste better after aging for a few days.
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Notes
If you don't have a food processor: You can crush the wafers in a zip-top plastic bag using a rolling pin. The pecans can be finely chopped by hand.
If the mixture is too dry, add another teaspoon or two of rum until the mixture is sticky enough to hold together.
If the mixture is too wet, crush some more Nilla Wafers and add a tablespoon as needed until the mixture is dry enough to handle.