In a medium bowl, whisk the flour, baking soda, and salt together. Set aside.

In a large bowl using a hand mixer or a stand mixer fitted with paddle attachment, beat the butter for 1 minute on high speed until creamy. Switch to medium-high speed and beat in the brown sugar and granulated sugar until completely creamed and smooth, about 3 minutes. (Here’s a helpful tutorial if you need guidance on how to cream butter and sugar.) Add the egg and beat until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed. Add the peanut butter and vanilla extract and beat until combined.

Add the dry ingredients to the wet ingredients and beat on low speed until combined, and then beat in the M&Ms. The dough will be very creamy and soft.

Cover and chill the cookie dough in the refrigerator for at least 1–2 hours, and up to 3 days.

Preheat oven to 350°F (177°C). Line large baking sheets with parchment paper or silicone baking mats.

Roll chilled cookie dough into balls, about 1.5 Tablespoons (around 30–35g) of dough each. For extra M&Ms on top, gently position some of the candies near the surface of the dough balls, or press a few additional M&Ms into the tops before baking. Arrange the dough balls on the baking sheets about 2 inches apart.

Bake for 12–15 minutes or until the edges appear set and are very lightly browned. The centers will still look very soft.

Allow cookies to cool on the baking sheet for 5 minutes. After 5 minutes, transfer to a wire rack to cool completely.

Cookies stay fresh covered at room temperature for up to 1 week.