Preheat oven to 350°F (175°C). Lightly grease a 9x9-inch baking pan or line it with parchment paper.
In a large bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the egg and vanilla extract until smooth.
In a separate bowl, whisk together flour, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring just until combined.
Fold in the chocolate chips and most of the mint M&Ms, reserving a handful for topping.
Press the dough evenly into the prepared baking pan.
Sprinkle the remaining mint M&Ms over the top and lightly press them into the dough.
Bake for 20–25 minutes, or until the edges are golden brown and the center is set.
Cool completely in the pan before cutting into bars.