With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. This isn't a lot of butter and it will get stuck on the sides of the bowl, so you may need to scrape down the sides of the bowl with a rubber spatula to really help get it creamed. Add confectioners' sugar, cocoa powder, 2 teaspoons of heavy cream/milk, and vanilla extract with the mixer running on low. Increase to high speed and beat for 3 minutes until it really comes together. Add another teaspoon of milk/cream if it looks a little too thick.
Remove only 2-3 cake balls from the refrigerator at a time. (Keep the rest cold!) Dip a lollipop stick about 1/2 inch into the coating, then insert into the center or the cake ball. Only push it about halfway - 3/4 through the cake ball. Dip the cake ball into the coating until it is completely covered. Make sure the coating covers the base of the cake ball where it meets the lollipop stick. Very gently tap the stick against the edge of the measuring cup to allow excess coating to drop off. Decorate the top with sprinkles and place upright into a styrofoam block, box (flip the box upside down and poke the lollipops into the bottom of the box) or even tall shot glasses work. Repeat with remaining cake balls, only working with some out of the refrigerator at a time. The cake balls must be very cold when dipping!
If you don't want to use a lollipop stick you can always use a toothpick to dip the cake balls into the coating but make sure to stick the toothpick all the way in so it can hold the weight of the ball while dipping.