To toast nuts: Preheat the oven to 350°F (177°C) and place the rack in the center of the oven. Place the pecans on a baking sheet and toast for about 8 minutes, stirring occasionally, until lightly browned and fragrant. Let the nuts cool completely then finely chop with a kinfe or use a food processor to pulse until finely chopped. Transfer the nuts to a large bowl, or the bowl of your stand mixer.
Process the vanilla wafer cookies in the food processor until finely ground. Add the crumbs to the nuts. To this mixture, add the powdered sugar and cocoa powder and stir well until combined.
In a measuring cup mix the corn syrup and rum until they come together into a single liquid form.
Lock the stand-mixer and set it to the lowest speed. Pour in the rum/syrup mixture into the dry ingredients until combined. Measure the granulated sugar into a shallow bowl and set aside.
Shape into 1-inch balls and roll them in the granulated sugar.Tip: You can refrigerate the dough for up to 30 minutes if the dough is too sticky. If your dough is too dry you can add a bit more rum, a ¼ tsp at a time, until it reaches the correct consistency. Store the finished rum balls in an airtight container in the refrigerator. They are best if made several days in advance of serving to allow the flavors to mingle. Serve them at room temperature.