Cream butter & sugars in the bowl of a stand mixer on high speed until light & fluffy.
Add the egg and vanilla and beat until well combined.
Add flour, cornflour, baking powder & salt and mix in well.
Add sprinkles and beat on low speed until combined.
Split dough in half and shape each half into a roll about 20cm long. Wrap in plastic wrap and chill in the fridge for at least 4 hours and even overnight. At this point, you can also put the rolls in the freezer.
Pre-heat the oven to 180 degrees C.
Remove the cookie roll from the fridge, unwrap and roll in sprinkles until well covered. If using frozen dough, allow to thaw in the fridge overnight.
Slice into 1-2cm thick slices, lay out on a lined baking tray and bake for 12-14 minutes, or until starting to go golden.
Allow to cool on the tray for 5 minutes, then transfer to a cooling rack.
Store in an airtight container for 1 week.