Preheat the oven to 325° and line two large baking sheets with silicone baking mats or parchment paper.
Cream together the melted butter and both the sugars. Add in the vanilla, egg and egg yolk and mix until well combined. In a separate bowl, combine the flour, baking soda, cornstarch and salt. Slowly mix the dry ingredients in with the wet ingredients. Mix just until combined. Fold in the chocolate chips and M&Ms.
Scoop 24 equal sized balls onto the prepared baking sheets. Bake in the preheated oven for approximately 12 minutes. Allow to cool before serving.