In a medium bowl, whisk together the flour, baking powder, corn flour and salt, and set aside.
Using a food processor or an electric mixer, beat/blitz together the butter and sugars until creamy, about a minute.
Beat in the egg and vanilla to combine.
Gradually add flour mixture to the butter mixture, mixing well after each addition.
Once the dough has come together, stir in the miniature M&M's.
Chill the dough for 30 minutes to 1 hour.
Preheat the oven to 180°C / 350°F / Gas mark 4.
Line 2 baking trays with greaseproof paper and set aside.
Once the dough has chilled, drop rounded tablespoonful’s of dough on the baking trays about 2.5cm apart.
Bake for 8-10 minutes, rotating the trays halfway through the bake-time.
Bake until light golden in colour and centres are just about set.
Cool the cookies on the sheets for 5-10 minutes before carefully transferring to wire racks to cool completely.
Store cookies airtight, at room temperature, for about 5 days.