Preheat oven to 350°F.
Prep: Spray a 9×13" pan with cooking spray.
Make the batter: Mix butter and brown sugar using a stand or a hand mixer until fluffy and smooth.
Next: Add in the eggs and vanilla and mix until smooth. Add in salt, baking powder and mix.
Then add in the flour, 1 cup at a time, mixing in between. Stir in M&Ms. The batter will be thick! This is normal.
Spread the cookie dough evenly into the baking dish.
Tip! If you find this hard with the batter being so thick, I either wet my hands and use my hand to press down the batter. Or use a spatula sprayed with nonstick spray to make it easier. Add a few extra candies on top for a wow factor.
Bake for 22-25 minutes, or until golden brown on top. Careful not to overbake or the cookie bars will be dry.
Wait to slice: Let the cookie bars cool in the pan for 15-20 minutes before cutting.
Store in an airtight container with a slice of bread to keep the bars fresh and soft 3-4 days.
