Place sweetened condensed milk, cocoa, and butter in a large glass bowl. Whisk together till blended.
Cook in the microwave for two minutes. Remove and whisk again. Continue cooking in the microwave for 4-5 more minutes, stirring often, till mixture is thick.
(If you aren't sure when it's done, drop a small spoonful into a glass of ice water for a few seconds. You should be able to form it into a ball.)
Chill for 2-3 hours, or till cold.
Roll in small balls, then roll in chocolate sprinkles.
Notes
You can make them larger if you prefer, they just won't make as many.