Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
Cream the butter and sugars in a mixing bowl until light and fluffy. Add egg and vanilla extract, and beat until smooth.
Whisk together the dry ingredients (flour, baking soda, salt) and slowly add them to the wet mixture.
Gently fold in the crushed cornflakes and any optional mix-ins like chocolate chips or coconut. Do not overmix — keep those flakes crisp!
Scoop spoonfuls of dough onto the baking sheet, spacing cookies 2 inches apart.
Bake for 10–12 minutes, or until edges are golden. Centers may look slightly soft — that’s perfect!
Let cookies cool on the tray for 5 minutes before transferring to a wire rack.