Line a 9x13" pan with parchment paper, so it hangs over the edges. Preheat oven to 350 degrees Fahrenheit.

In the bowl of an electric mixer, cream together butter, light brown sugar, granulated sugar, vanilla and eggs, scraping the sides down with a spatula in between.

In a separate bowl, stir together flour, salt, and baking soda. Add flour mixture to wet mixture and mix until incorporated.

Reserve 1 c. of cookie dough. Press the remaining cookie dough into parchment-lined pan. Spread marshmallow creme over the top. Grab bits of the reserved cookie dough and press them flat in your hand, then lay them on top of the marshmallow creme. They won't cover the entire top, but piece them together as best you can.

Sprinkle and press M&M's into the batter on top, then bake for approximately 22 minutes, or just until set. Don't overbake! Let cool completely, then carefully lift the cookie bars from the pan with the parchment paper. Slice and serve.