The BEST soft, thick, and chewy M&M cookie bars for any occasion! Bake in a standard 9x13 pan for a quick & easy after-school treat, birthday cookie cake, or add seasonal M&M's to match any holiday!
Preheat oven to 350 degrees.
In a small microwave-safe bowl, melt the butter for 20-30 seconds until JUST melted. You want it fully melted down without any chunks.
Add in white sugar and brown sugar and use a spatula to stir until combined (* do not use a mixer!). Add in egg and vanilla, then mix. Next, add in baking soda, baking powder, and salt and mix again. Add in flour and mix until all the flour disappears into the dough.
Add the chocolate chips and M&M's into the dough and mix (if desired, press a few extra M&M's on top for aesthetics). Immediately proceed to pressing into pan (you don't want the dough sitting out or it will set too much with the melted butter).
Grease a 9x13 metal pan. Drop dough into chunks then use a spatula or your hands to press down the dough into a smooth layer (being sure to get all the corners).
Bake at 350 degrees for 14-15 minutes - do NOT overbake or your bars will not be soft. You want the cookie cake to be slightly brown on the top and look slightly underbaked (my oven was perfect at 14 minutes). Remove from oven and place entire pan on a cooling rack. Allow the cookie cake to cool completely before removing.
Freezing/Make Ahead Tip: I don't typically like freezing chocolate chip cookies because I feel like they get dry and not as chewy in the freezer. If you do need to make this in advance, wrap the cut baked bars in plastic wrap then place in a airtight bag or container. When you’re ready to eat, let it thaw to room temperature (or just eat frozen…my fave!).
If you’ve got a good airtight plastic container, that will also work for freezing…although I find that they absorb a little more of the freezer taste when they aren’t wrapped in plastic wrap first!