Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with peanut butter and both sugars until fluffy.
Add in the egg and vanilla extract and mix until smooth. Add in the flour mixture, and mix until it is fully combined. Fold in the chocolate chips and M&Ms.
Evenly press the cookie dough into a greased round 9-inch pie plate. Bake for 20-25 minutes.