Preheat oven to 160°C/325°F. Whisk sugar and egg together in a bowl until well combined. Add cornflakes and mix until evenly coated with the mixture.
Scoop half tablespoon scoops of batter onto a silicone baking mat-lined baking sheet, leaving at least 5 cm/2 in. of space between the mounds of batter. Flatten each mound and sprinkle with a bit of flaky salt.
Transfer sheets to the oven and bake at 160°C/325°F for approx. 15 min., rotating the pans midway through baking. Let the cookies cool for at least 10 min. Serve and enjoy!