1. Preheat your oven to 325°F. Line baking sheets with parchment paper to prevent sticking. This simple step ensures that your cookies come off the sheets easily.
2. In a large bowl, combine all purpose flour, cocoa powders, baking soda, and kosher salt. Use a whisk to stir, evenly distributing the dry ingredients and aerating the flour at the same time. Set this dry mixture aside for later.
3. In a large measuring cup, melt together 4 oz (1 stick or 1/2 cup) of butter and milk or semisweet chocolate chips in the microwave. Heat in increments of 30 seconds, stirring after each interval until everything is smooth and combined. Be careful not to overheat it.
4. In a mixing bowl, beat the remaining 4 oz of butter together with granulated sugar. Scrape the bowl often to ensure everything is mixed well. Incorporate the eggs, one at a time, mixing on medium speed after each addition to incorporate air.
5. Add vanilla extract to the mixture and continue mixing on medium speed until fully combined.
6. Lower the mixer speed and gradually pour in the chocolate-butter mixture. This step is crucial; adding it slowly prevents cooking the eggs with the warm chocolate.
7. Once the chocolate is fully incorporated, begin adding the dry ingredients in increments, mixing on low speed to combine.
8. Gently fold in both types of M&Ms and white chocolate chips by hand, ensuring they are evenly distributed throughout the dough.
9. Using a medium-sized cookie scoop, drop 6-8 dough balls per baking sheet, spacing them evenly apart. They will spread during baking.
10. Bake each tray in your preheated oven for 12 to 14 minutes. The cookies will puff up. If you prefer them flatter, press each cookie down gently with the flat back of a spatula immediately after removing them from the oven.
11. Allow the cookies to cool on the baking tray for a few moments before transferring them to a wire rack to cool completely.
