Preheat oven to 325 degrees and grease a 9 x 13 baking pan.
In a small bowl, melt the butter in the microwave. Set aside to allow to cool slightly.
In a medium bowl, combine the flour, salt, and baking soda.
Add the brown sugar, white sugar, egg, and vanilla extract to the melted butter.
Once combined, slowly add in the flour mixture and stir until just combined.
Fold in the chocolate chips and 1/2 cup of M & M's.
Spread dough evenly into prepared pan.
Sprinkle remaining M & M's on top of spread dough and lightly press into dough.
Place pan in preheated oven and bake for about 25-30 minutes.
Remove from oven and allow to cool slightly before slicing and serving!