Preheat the oven to 350 degrees and prepare a cookie sheet lined with parchment.
Combine flour, baking soda, and salt with a whisk and set aside.
In the bowl of an electric mixer fitted with the paddle attachment, on medium speed, beat melted butter and both sugars until well combined. the bowl of an electric mixer fitted with the paddle attachment, combine melted butter and both sugars (on medium speed) until well combined.
Reduce speed to low and add egg, egg yolk and vanilla. Beat until well mixed, about 1 minute.
Slowly add the dry ingredients and mix until just combined.
Stir in chocolate chips and M&Ms.
Using a medium ice cream scoop, scoop dough balls and place on baking sheet.
With the back of a wooden spoon, gently press down on the cookies to make them flat instead of domed.
Bake at 350 for 10-12 minutes, or until the edges are golden brown.
Let cookies cool on the baking sheets.