Combine flour, baking soda and salt in a large bowl and whisk to blend evenly.

Combine softened butter, brown sugar and granulated sugar in a large bowl (or in the bowl of an electric stand mixer fitted with the flat beater/paddle attachment) and beat with an electric hand mixer on medium speed for 1 ½ to 2 minutes until the mixture looks somewhat light and fluffy but still grainy. You can also mix by hand with a wide spatula (my preferred method).

Add eggs one at a time and beat until incorporated after each addition, about 20 seconds per egg. Mix in vanilla extract until evenly blended. Gradually add flour mixture in two additions, mixing on low speed for no more than 10 seconds after each addition, or fold it in with a spatula. Some flour should still be visible since you will continue to mix the dough when you add the chocolate and M&Ms. Add chopped chocolate or chocolate chips and Christmas M&Ms and mix on low speed just until evenly distributed. The dough will be moist. Turn dough out onto a large piece of plastic wrap and wrap well without pressing it down or compacting it. Refrigerate for 8-36 hours.

When ready to bake, preheat your oven to 350°F and line two large cookie sheets with parchment paper.

Press a few extra chocolate chips or M&Ms on top of each dough ball, flatten them slightly and bake for 9-11 minutes until puffed, golden around the edges but still soft in the middle. Do not over-bake as they will continue to cook on the hot baking sheet. Transfer baking sheets to a wire rack and let cookies cool on the trays for 2 minutes before carefully transferring individually to a wire rack to finish cooling.