Microwave butter for 45 seconds (it will be partially melted). Note: If your butter is not straight from the fridge and has already softened to room temperature, skip microwaving.
1 stick butter
Cream together butter and brown sugar on medium speed until mixture gets light and fluffy, about 2 minutes.
¾ cup brown sugar
Reduce mixer speed to low. Add egg and mix until fully incorporated, about 1 minute.
1 egg
Mix in vanilla, salt, and baking soda.
½ tsp vanilla, ¼ tsp kosher salt, ½ tsp baking soda
Add flour a half cup at a time and mix until all flour is incorporated.
1¼ cups all purpose flour
Mix in chocolate chips and M&Ms. (Optional: Reserve some of the M&M's to press a few on top of each cookie before baking. This will make sure the M&M's are visible in every cookie.)
½ cup semi-sweet chocolate chips, ¾ cup M&M's candy
Using a cookie scoop or spoon, form dough into balls and place onto a greased cookie sheet.
Bake for about 10 minutes or until cookies are golden brown.
Let cookies cool on the cookie sheet for about 5 minutes before removing from the cookie sheet.
Notes
You can substitute more M&Ms if you don't have chocolate chips.