Preheat oven to 170°C, and line oven trays with baking paper. Beat butter and sugars in an electric mixer until pale and fluffy (3-4 minutes). Beat in egg, then add remaining ingredients except cornflakes and mix until smooth.
Preheat oven to 170°C. Divide dough into 12 even balls and, with damp hands, roll balls in cornflakes. Place on trays, leaving 5cm-6cm between each to allow for spreading, and bake until golden (15 minutes). Cool on tray or on a rack. Cookies will keep for up to a week in an airtight container.