Beat together the butter and brown sugar until well combined. Beat in the eggs and vanilla. Mix together the flour, starch, soda, and salt. Add to the butter mixture and mix just until combined. Add the chocolate chips and the M&M's (save some for the tops if you want to). Throw the dough in the refrigerator (over night is best, but at least until firm).
Preheat oven to 375ºF. Roll cookies to desired size and place on ungreased baking sheet a couple inches apart. Bake cookies for about 7-10 minutes or until the edges are golden brown. If you saved some M&M's, press them into the tops of the warm cookies.