Preheat oven to 350F. Using an electric hand mixer or stand mixer with paddle attachment, cream together butter and brown sugar. Beat in corn syrup, egg, and vanilla.  Next, mix in salt, baking soda, and cornstarch. Add flour; mix until fully combined. Add in M&Ms by hand or with stand mixer on low.

Using a nonstick cookie sheet and/or parchment paper, scoop dough out (I use a 2 T ice cream scoop), leaving 2" between cookies. Bake for 10 minutes, or just until cookies are no longer "shiny." Remove from oven and allow to cool on sheets for 4-5 minutes. Remove to a cooling rack and allow to cool further. Cookies will appear very soft (borderline undercooked) inside initially, but will end up perfect after they are allowed to cool further.