Whisk together the flour, baking soda, and salt; set aside.
Beat the butter, granulated sugar, and brown sugar together until light and fluffy.
Beat in the eggs one at a time, then mix in the vanilla.
Gradually mix in the dry ingredients until just combined.
Fold in most of the M&M's, reserving a handful for the top.
Press the dough into a parchment-lined 9x13-inch pan and top with reserved M&M's.
Bake at 350°F for 25 to 30 minutes, until edges are golden and the center is set.
Cool completely in the pan before slicing into squares.