Place wafers in a food processor. Pulse until finely ground. They should measure about 2½ cups (625 mL). In a medium bowl, stir liqueur, butter, corn syrup, 1 cup (250 mL) icing sugar and crumbs until evenly mixed. Stir in white chocolate and pecans.
Line a tray with waxed paper. Shape mixture into 1-in. (2.5-cm) balls and place on tray. In a small bowl, stir 2 tbsp cocoa powder. Roll balls in cocoa mixture until coated. Store in an airtight container with wax paper between layers. Refrigerate at least overnight or up to 2 weeks. Flavour improves as they sit.