Using electric mixer, beat butter, icing sugar, granulated sugar, vanilla and salt for about 3 minutes or until light and fluffy. Mix in flour just until combined; fold in 1/2 cup (125 mL) cereal.
Divide dough in half; shape each portion into two logs, about 4 inches (10 cm) long and 1 1/2 inches (4 cm) in diameter. Wrap each log in plastic wrap; refrigerate for about 1 hour or until firm.
Preheat oven to 325°F (160°C). Line 2 baking sheets with parchment paper. Cut each log into 1/4-inch (5 mm) thick slices; place about 1 inch (2.5 cm) apart on prepared baking sheets. Top with remaining cereal; press gently to adhere to dough.
Bake in batches for 12 to 15 minutes or until lightly golden on the bottom. Let cool completely on rack.
Alternatively, roll out dough between 2 sheets of parchment paper into 1/4-inch (5 mm) thickness. Use holiday cookie cutters to cut shortbread into festive shapes.
For a chocolate-mint variation, substitute mint extract for vanilla extract.