In a large mixing bowl, cream the butter and sugar for about 4 minutes or until creamy and pale. Add the vanilla and beat again. Then add the egg and beat until smooth. Add the peanut butter and beat until combined.
In a separate bowl, sift the flour, baking soda, cornflour and gently whisk together. Then add the dry ingredients to the wet and gently fold together. Add the rolled oats, M&M's and chocolate chips and stir until combined.
Cover the bowl with plastic wrap and refrigerate for at least one hour.
Line two oven trays with baking paper. Preheat the oven to 350 F.
Grab cookie dough from the fridge. The dough will still be a bit sticky but it should be firm enough to roll balls using your hands.
Line the balls (approximately 1 heaped tablespoon) of cookie dough onto the prepared trays and then flatten slightly using your hand or a spatula, as they wont spread out as much as a normal cookie would. Press a few extra M&M’s and chocolate chips on top if desired.
Bake the cookies for approximately 12 minutes or until golden brown on the edges. Leave the cookies to cool slightly before gently moving to a wire rack to cool completely.