Microwave the butter for about 40 seconds. Butter should be completely melted but shouldn't be hot.
In a large bowl, mix butter with the sugars until well-combined.
Stir in vanilla and egg until incorporated.
Add the flour, baking soda, and salt. Mix until just combined. Dough should be soft and a little sticky but not overly sticky.
Stir in M&Ms.
Scoop out 1.5 tablespoons of dough (medium cookie scoop) and place on baking sheet.
Bake for 7-10 minutes, or until cookies are set. They will be puffy and still look a little underbaked in the middle.***
*Be sure to fluff and then scoop and sweep to measure the flour. Too much flour will result in a dry or doughy cookie.** Or 1/4 teaspoon table salt***Don't over-bake the cookies, or you won't end up with soft cookies. Slightly underbaking the cookies will give you soft cookies that stay soft. ****Nutrition facts are estimates.