Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt and baking soda. In a large bowl, mix together butter and both sugars 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 butterscotch chips and M&Ms. Use a cookie scoop to place cookies onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.