Add the butter, brown sugar and granulated sugar to a large mixing bowl. Using an electric hand mixer or a stand mixer, cream the butter and sugars together for 1-2 minutes, until pale and creamy. NOTE: If using a stand mixer, you may need to scrape the sides of the bowl once or twice.
Add the egg and vanilla extract to the bowl; Using medium speed, beat the ingredients, scraping down the insides of the bowl as necessary. Continue mixing until ingredients are well combined with the butter mixture.
Add the flour and baking soda, and beat on low speed until the ingredients are just combined. Add the M&Ms and chocolate chips to the bowl, then use a mixing spatula or wooden spoon to stir them into the dough.
Cover the top of the bowl with plastic wrap and chill the dough in the fridge for at least 1 hour.
When the dough is cold, preheat your oven to 325°F/160°c. Line 2 baking sheets with parchment paper and set aside.
Portion the dough using a 1.5 tablespoon cookie scoop and roll into a ball (it doesn’t have to be perfectly round). Place the dough balls on the prepared baking sheets.
Bake the cookies in the preheated oven for 10-12 minutes, or until the edges are set and light golden brown. The cookies will still be soft at this stage, but will firm up as they cool.
Allow the cookies to cool completely on the baking sheets.
Notes
Avoid overmixing the dough. Too much mixing can cause the cookies to be tough and crumbly.
Take time to thoroughly chill the dough. If the cookie dough isn't cold enough, it will spread out too much in the hot oven, causing the cookies to be thin and crispy.
Remove the cookies from the oven when the tops aren't quite set. As they cool on the hot baking sheet, the cookies will continue baking.
If the M&Ms crack open in the hot oven, wait until you pull the cookie sheet out of the oven. Then, use the tips of your fingers to gently press down on the warm candies.