In a small sauce pot combine the sweetened condensed milk, bittersweet chocolate, cocoa powder and salt. Place the pot over medium high heat and bring to a gentle boil, stirring constantly with a wooden spoon. Reduce heat to medium-low and cook 10 to 15 minutes, stirring constantly, until mixture is thick and shiny and starts to pull away from bottom and side of sauce pot. The mixture is going to get thick. Remove from the heat and stir in the butter and vanilla until melted and smooth. Pour mixture into glass bowl, cover and place in the fridge for at least 1 1/2 hours or overnight.

Once chilled place the sprinkles (or whatever you are using) in a shallow bowl. Rub some softened butter over hands and scoop out rounded teaspoon of the chocolate mixture and roll into 1 to 1 1/2 inch balls. Roll each ball in the chocolate sprinkles (or whatever you are using). Place on a baking sheet lined with parchment paper.

Store tightly covered at room temperature up to 2 days or in the refrigerator up to 1 week.