1. Preheat oven to 175°C
2. Add the butter, sugar and egg to a mixing bowl and whisk to combine. Then add the plain flour, baking soda and baking powder and whisk again.
4. Fold in the M&MS and pour the cookie batter into a greased skillet pan.
5. Bake for 18 minutes, until the cookie is brown on top but still slightly squidgy underneath.