First, stat by preheating the oven to 350F then line a baking sheet with parchment paper and set aside.
Cream the butter, granulated sugar, and the brown sugar until light and fluffy, about 3 minutes. Add in the egg yolk, and the vanilla and mix again until fluffy and well incorporated.
Add in the flour, cocoa powder, baking soda, and salt and mix just until combined. Don't over mix. Using a small cookie scooper, scoop out 14 equal cookie dough balls. Roll them between the palm of your hand then roll one side in the sprinkles. Set on the prepared baking sheet. Repeat with the rest of the cookies and bake for 10 minutes in the preheated oven.
Pull out the baked cookies, let them cool on the baking sheet for 5 minutes then transfer to a cooling rack. Enjoy with a tall cup of cold milk.