Combine flour, baking soda, corn starch, and salt in a small bowl. Beat butter, granulated sugar, brown sugar and vanilla extract in a large mixer bowl until cream. Add eggs, one at a time, beating well after each addition. Gradually beat in flour mixture. Stir in old fashioned oats. Drop by rounded tablespoon onto ungreased baking sheets.
Bake for 5 minutes. Remove from oven, and add M&M's on top of each cookie. Return baking sheet to oven, and cook for another 3 - 5 minutes, depending how big the cookies are.
Remove onto wire racks and cool completely.
Recipe by Farmhouse 1820 at https://farmhouse1820.com/mm-cookie/