In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt, and set aside.

In a medium saucepan (light-colored interior if you have one), melt the butter over medium-high heat until completely melted and bubbling. Lower the heat to low-medium, and use a heat-proof spatula to stir the butter constantly. It should take about 3-5 minutes to start to brown. Once brown bits start to form on the bottom of the pan, remove it from the heat and transfer the butter into a large glass bowl. Immediately stir in the milk and vanilla extract to stop the butter from burning. Allow the butter to cool for 10-15 minutes, or until it's still liquid, but the bowl feels mostly cooled.

Stir in the sugar and brown sugar until well combined. Then add in the egg, and again stir until the mixture is combined and the egg is no longer visibly showing any streaks. Add in the dry ingredients, and mix until you've got a thick dough formed. Then, finally add in the chopped chocolate.

On a small baking sheet or plate lined with parchment paper, scoop your cookie dough into about 2-tablespoonful-rounds. You should end up with 12-15 cookies in total. Roll the tops of each round into the sprinkles to coat the tops. Cover the cookies in plastic wrap, and chill in the fridge for at least 1 hour, and up to 48 hours.

After chilling the dough, you can either transfer the dough to a freezer safe container and freeze for 3-4 months before baking, or you can bake them now! Preheat your oven to 350 degrees and line a large baking sheet with parchment paper. Place the cookie dough about 2 inches apart on your baking sheet. Bake for 12-14 minutes, or until lightly golden around the edges. Cool for 5-10 minutes before eating them warm!