Pre-heat oven to 350°F and prepare a large baking sheet with parchment paper. Microwave butter for approximately 30 seconds until slightly melted.
Using an electric hand mixer, beat together melted butter, granulated sugar, and brown sugar until combined. Add egg and vanilla extract, beating again until combined. Add salt, baking soda, and flour. Mix together until just combined.
Stir in M&M candies, and chocolate chips if using. Place a 1 1/2 to 2 Tablespoon dollop of dough on the baking sheet. Bake for 11-12 minutes, until edges just become golden brown. Let cool on pan for 2-3 minutes until set enough to transfer to a cooling rack.
I've used a regular whisk to mix up the cookies before, and it does require a little more muscle but totally works! Use a strong spatula to stir in the dry ingredients.Nutrition calculation is based on an estimate and does not include the optional chocolate chips.