Combine the sweetened condensed milk, chopped bittersweet chocolate, cocoa powder, and salt in a small saucepan.
Place the pan over medium-high heat and stir constantly until it comes to a gentle boil.
Reduce heat to medium-low and cook, stirring constantly, for 10 to 15 minutes until the mixture is thick, shiny, and starts to pull away from the bottom and sides of the pan.
Remove from heat and stir in the butter and vanilla until fully melted and smooth.
Pour the mixture into a glass bowl, cover, and refrigerate for at least 1½ hours or overnight until fully chilled and firm.
Place the sprinkles or chosen topping in a shallow bowl and lightly rub softened butter on your hands.
Scoop rounded teaspoons of the chilled chocolate mixture and roll into 1 to 1½-inch balls, then roll each ball in the topping to coat.
Place finished brigadeiros on a baking sheet lined with parchment paper.