Butter the bottom only of an 8 x 8 inch square pan and line. with parchment paper, allowing some parchment to hang over the sides. Set aside.
In a medium bowl combine the flour, salt, baking powder and baking soda. Set aside.
In a large bowl, using a wooden spoon or rubber spatula, combine the brown sugar and melted butter until well combined. Add in the egg and vanilla, mixing until both are incorporated.
Slowly add the flour mixture to the brown sugar mixture and stir until all the flour is incorporated. Mix in ½ cup of the Valentine M&M's.
Spread batter into prepared pan and sprinkle with remaining M&M's.
Bake for 25 to 30 minutes or until a toothpick inserted in the center comes out clean.