Melt the butter in a small frying pan, over medium-low heat. After about 4 minutes, the butter will begin to foam. Gently whisk the butter constantly from now on. The butter will continue to bubble and foam for another 4-5 minutes. Once it begins to turn brown on the bottom of the pan and smell nutty and caramel-ly, remove the pan from the heat, and pour the hot brown butter into a heat proof bowl. Allow to cool slightly for about 10 minutes.

In the bowl of a stand mixer fitted with a paddle attachment, or a large bowl with an electric handheld mixer, mix the butter, brown sugar, and granulated sugar for 2 minutes. Beat in the egg, egg yolk, vanilla extract, and sour cream for another minute.

Add the flour, salt, and baking soda to the bowl, and mix on low speed until just combined. Add the M&Ms and chocolate chips and mix in gently, by hand.

Using an ice cream scoop, portion out dough balls onto a large plate. Cover with plastic wrap, and refrigerate for at least 2 hours.

Preheat oven to 375 degrees F. Remove cookie dough balls from the fridge, and place on a baking sheet lined with parchment paper. Bake for 10-12 minutes, rotating the pan 180* halfway through. The cookies should be lightly golden brown, and just set. Don’t overbake! Allow to cool on the baking sheet for 5 minutes, then transfer to a wire rack or brown paper bag to cool completely. Enjoy!