Preheat the oven to 375.
If you forget to remove your butter from the fridge ahead of time, you can slightly soften it in the microwave. You don't want them super soft or melted, so make sure not to over-cook them! I put the two sticks in my microwave for 10-15 seconds and that gets them slightly soft (which is perfect!). You just want to make sure your butter isn't too soft or else the dough is more likely to spread.
Using the paddle attachment of your stand mixer, cream butter & sugar together on medium speed for 2-3 minutes.
While the butter & sugars are creaming together, pour 3 c flour (leave that other 2 tbsp for later), baking soda, and salt in a separate bowl and gently mix together with a whisk, and then set aside.
Add in vanilla & egg to the creamed butter and sugar until egg is combined (about 15-20 seconds), scraping the bowl to make sure everything evenly mixes together.
With the mixer on low, slowly add in the flour mixture to the creamed butter, and mix together until just combined.
Gently add in the chocolate chips.
Now, for a silly-but-always-works trick for helping those cookies not to spread: sprinkle in the remaining 2 tbsp of flour into the cookie dough and quickly mix together - just a few rotations around the bowl!*
Roll out your cookie dough - the dough balls should be about 2 inches in diameter. After you roll each one, add some Christmas M&Ms to the top and sides of the dough ball - maybe 6-8 M&Ms.
Once you've rolled out 12 and placed them on your baking pan, bake on the center rack in the 375 degree preheated oven for 9 minutes. (See my note on baking pans)
While those cookies are baking, roll out the other half of the dough, add M&Ms, and place on a second baking sheet.
Remove the baked cookies from the oven, and allow them to sit on the baking pan for 5-10 minutes before transferring them on a cooling rack. Place the second pan of cookie dough balls in the oven & bake.
Remove from oven, allow to cool on wire rack, and then enjoy!