Pre-heat oven to 350F degrees. Line a 9x9 baking pan with foil (or parchment) and spray with non-stick cooking spray. Set aside.

In a medium bowl, combine the graham cracker crumbs and salt. Pour in the melted butter and stir until well combined and all the crumbs are evenly coated with butter. Press graham cracker crust into bottom of prepared pan. Bake for 15 minute or until crust is lightly brown. Remove from the oven and set aside.

Prepare brownie mix as directed on the back of the box. Stir in about 1 cup M&M's and reserve the rest for the top of the brownies. Pour brownie mix on top of crust. Bake for approximately 45-50 minutes or until a toothpick inserted into the center comes out clean.

Turn the oven to broil. Arrange the marshmallows over the top of the baked brownies. They don't need to touch. Place the pan on the lower rack of the oven and broil for 1 to 2 minutes, until the marshmallows are melted and light brown on top — 30 seconds to 1 minute for golden marshmallows, and 1 1/2 to 2 minutes if you prefer more charred marshmallows. (Check every 20 seconds.)

Remove from the oven and immediately sprinkle the tops with the remaining M&M's (press M&M's into marshmallows slightly), so they will stick to the melted marshmallows. Let the brownies cool completely. To remove, gently lift the brownies out of the pan by using the extra foil on the sides. Pull the foil away and cut into squares.

Best served immediately. Store leftovers in the pan covered with foil for 3-4 days.