Preheat your oven to 350 degrees F, and then line a cookie sheet with parchment paper.

In a large mixing bowl, cream together the unsalted butter and light brown sugar using an electric mixer.

Next, add a room-temperature egg, peanut butter, and vanilla extract.

Beat all the ingredients together until they are well combined.

In a small bowl, whisk together the flour, baking soda, and salt.

Gradually add the dry ingredients to the wet ingredients. Mix them until they are well combined.

Fold in the peanut M&M's, making sure they are evenly distributed.

Cover the bowl with a lid or plastic wrap, and then chill the peanut butter cookie dough in the fridge for at least 30 minutes.

Using a large cookie scoop, scoop out the dough and place each dough ball on the prepared baking sheet. Be sure to place them about 2 inches apart.

Bake your cookies for 10 minutes or until the cookies are golden brown around the edges.

Allow the chewy peanut butter M&M cookies to cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.

Serve and enjoy!