In a large bowl, cream together caster sugar, dark brown soft sugar and butter. Add vanilla and eggs; mix well. Sieve together flour, bicarboante of soda and salt; stir into butter mixture. Finally, mix in rolled oats. Refrigerate dough for 1 hour.
Preheat oven to 180 C / Gas 4.
Roll dough into 5cm balls. Place balls on a baking tray, Press down slightly and insert a lollipop stick into each one. These will spread a bit so don't put too many to a tray. Bake 8 to 12 minutes or until golden brown. Cool 1 minute then arrange M&Ms into a happy face design. The cookies will be chewy, but firm.