In a saucepan over medium heat, combine the sweetened condensed milk, cocoa powder, and butter.
Stir continuously until the mixture thickens and starts to pull away from the sides of the pan (about 10 minutes).
Remove from heat and stir in vanilla extract if using.
Let the mixture cool slightly, then refrigerate for 1 hour until firm enough to handle.
Scoop and roll the mixture into small balls (about 1 inch).
Roll each truffle in chocolate sprinkles to coat.
Place on a plate or tray and chill for at least 30 minutes before serving.
