Lightly grease a heatproof plate or small baking dish with 1 teaspoon butter. Set it aside. Place the chocolate sprinkles in a shallow bowl or plate.

Add the sweetened condensed milk, cocoa powder, 2 tablespoons butter, and salt to a medium heavy-bottomed saucepan. Whisk until the cocoa is fully incorporated and the mixture is smooth.

Cook over medium-low heat, stirring constantly with a silicone spatula or wooden spoon. Scrape the bottom and corners of the pan continuously so the mixture does not scorch.

After about 10 to 12 minutes, the mixture will be very thick and glossy. It is ready when you can drag the spatula across the bottom of the pan and the trail stays open for 2 to 3 seconds before slowly closing.

Immediately scrape the mixture onto the greased plate and spread it into an even layer. Let it cool at room temperature until no longer hot, then cover and refrigerate until firm enough to roll, about 1 hour.

Lightly butter your hands. Scoop about 1 tablespoon (15 grams) of the chilled mixture and roll it between your palms into a ball. Roll the ball in chocolate sprinkles and place it in a paper candy cup or on a parchment-lined plate.

Repeat with the remaining mixture, lightly buttering your hands as needed. Serve slightly cool or let the brigadeiros stand at room temperature for 10 to 15 minutes for their softest texture.