Find out what the secret ingredient is in these perfectly thick and soft peanut butter m&m cookies! The texture of these cookies is incredible! 

Thick and Soft Peanut Butter M&M Cookies on a sheet pan

I have been hearing about people putting cornstarch in their cookies for some time now. Everyone claiming that it makes the cookies ridiculously soft and fudgy.

This has been nagging at me. Everyone who’s tried it swears by it, but nobody ever talks about why! So I finally decided to give it a go, and of course, it does work! Surprisingly well. These  cookies turned out to be some of the softest, most dense and fudgy cookies I have ever baked.

I was thinking about the science behind this the other day and trying to do some research as to why cornstarch has this effect in baked goods. And while I haven’t found one definitive answer, this is what I’ve concluded: We know that cornstarch and flour are both starches or course, and their main purpose in baking is the hold structure.

However, flour has gluten in it and this gluten reduces the thickening power of flour. Therefore, cornstarch is about twice as powerful of a thickening agent compared to flour. By adding some cornstarch to the mixture you can have a dough that has far more liquid and fat in it compared to the flour and it will still hold it’s shape.

Liquid and fat create moistness and softness in a cookie, so this is my theory. The cookie will be able to stand tall and hold it’s shape because of the thickening power of cornstarch while having a high ratio of fat and liquid. I’m sure there are more in depth sciency things behind it, but I love having these epiphanies!

Upclose view of Thick and Soft Peanut Butter M&M Cookies on a sheet pan

My variation on this cornstarch cookie formula is a really vanillay cookie filled with peanut butter m&m’s. It feels like something you would get from a good bakery. You know? One of those cookies that is unexplainably softer than anything you have ever been able to make at home.

Huge and fudgy and thick! It’s taking m&m cookies to the next level. To the level where they will get both kids and adults to love them. Not that adults don’t love regular m&m cookies, but these are fancier!

Thick and Soft Peanut Butter M&M Cookies on a sheet pan
Yield: 12-14 Cookies

Ultra Thick and Soft Peanut Butter M&M Cookies

Prep Time 18 minutes
Cook Time 12 minutes
Total Time 30 minutes


  • 1 1/2 sticks unsalted butter, softened
  • 1 cup light brown sugar, lightly packed
  • 1 large egg, room temp
  • 1 TBSP vanilla extract
  • 2 cups all-purpose flour
  • 1 TBSP cornstarch
  • 1 tsp baking soda
  • 1/2 tsp kosher salt
  • 11 oz package peanut butter m&m's


  1. In the bowl of a stand mixer fitted with a paddle attachment (or large mixing bowl with an electric mixer) cream the butter, sugar, egg, and vanilla, beating on medium-high speed until light and fluffy. About 3 minutes.
  2. Scrape down the sides of the bowl, and add the flour, cornstarch, baking soda, and salt. Beat on low speed until just combined. Scrape down the bowl as needed.
  3. Add the M&Ms, and beat on low speed until just combined.
  4. Use a large cookie scoop (or 1/4 cup measuring cup) to form large dough mounds. Place about 6 cookies each on an ungreased baking sheets, or sheets lined with parchment or a silpat. Use your hand to flatten the dough just slightly, keeping the cookie dough still very tall.
  5. While the oven is preheating to 350F, place the cookie sheet in the freezer for about 5 minutes before baking. This will chill your dough and allow the outside to set before spreading too much while baking.
  6. Bake at 350F for 11-12 minutes, or until edges have set and tops look set. The middle will still look undercooked. Cookies firm up as they cool.
  7. Let cookies cool on the baking sheets for 10 minutes before serving warm or moving to a cooling rack to completely cool.

Nutrition Information:

Amount Per Serving: Calories: 422