Preheat your oven to 350°F (175°C) to ensure even baking and perfect golden-brown cookies.
In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt to evenly distribute the leavening agent and salt.
In a separate bowl, cream together the very soft unsalted butter, granulated sugar, and brown sugar with a hand mixer or whisk until light and fluffy, about 2-3 minutes.
Add the egg yolk, milk, and vanilla extract to the butter-sugar mixture. Beat until well combined and smooth.
Gradually stir the dry ingredient mixture into the wet ingredients until just combined; a few flour streaks are okay to avoid overmixing.
Gently fold in the mini M&M's or chocolate chips using a rubber spatula to evenly distribute them throughout the dough.
Drop heaping tablespoons of dough onto a parchment-lined baking sheet, spacing about 2 inches apart, making 6-8 cookies depending on size.
Bake in the preheated oven for 9-11 minutes, until edges are set but centers remain soft; cookies will continue to cook slightly after removal.
Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Enjoy warm or store for later!