Place 1 (14-ounce) can sweetened condensed milk, 1/3 cup cocoa powder, 2 tablespoons unsalted butter, and 1/4 teaspoon kosher salt in a medium skillet.
Heat over medium heat, stirring slowly with a heatproof spatula and breaking up any large lumps by gently pressing on them with the spatula.
Continue to cook, stirring frequently and scraping the bottom and sides, until thickened significantly and pulling away from the sides of the pan to create a fudgy mass in the middle, 8 to 10 minutes.
Transfer to a plate, cover with plastic wrap, and freeze until completely chilled, about 30 minutes.
Spread 1/2 cup chocolate sprinkles out on a plate.
Scoop out heaping tablespoons of the chocolate mixture and roll into balls (about 1 1/4-inch wide).
Roll in the chocolate sprinkles until completely coated, dampening your hands lightly if the chocolate mixture starts to stick to them.
Place on a plate or platter.