Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with sugar until fluffy. Add in egg and vanilla extract, and mix until smooth.
Add in the flour mixture, and mix until it is fully combined. Fold in the M&Ms and chocolate chips. Evenly press the cookie dough into a greased 8-inch square baking dish. Bake for 20-25 minutes.