In a medium sized mixing bowl, melt the butter. Whisk in both sugars, followed by the egg yolk, vanilla, and milk.
Add in the flour, corn starch, baking powder, baking soda, and salt. Fold the dry ingredients into the dough until just a few flour streaks remain.
Add in the m&m's and chocolate chips and fold the mixture together just until no flour streaks remain.
Pour the dough into the 6" cake pan and top with additional m&m's if desired. Bake for ~20 minutes, or until the edges begin to turn golden brown. The middle may slightly jiggle but that's okay - it will continue to cook even after it's taken out of the oven.