In a large bowl, combine the melted butter and granulated sugar. Whisk to combine.
Add the egg and vanilla extract and whisk until well combined.
Next, add the flour and baking soda and stir just until no large lumps of flour remain.
Add the M&Ms and mix just until they’re combined - you don’t want to overmix!
Cover the bowl and transfer the cookie dough to the fridge to chill for 30 minutes.
While the cookie dough chills, preheat your oven to 350°F and line two large baking sheets with parchment paper or silpat mats.
Once the dough has chilled, scoop two-tablespoon sized cookies onto the baking sheets, making sure they’re at least 2 inches apart.
Bake the cookies for 12-14 minutes, or until the edges are golden brown. Allow them to cool fully before removing them from the tray - they will set as they cool.
Transfer the cooled cookies to an airtight container and store at room temperature for up to three days.