In a medium saucepan, combine the sweetened condensed milk, chopped chocolate, cocoa powder, espresso powder (if using), and salt.
Place the pan over medium-high heat and stir constantly until the mixture comes to a gentle boil; then reduce heat to medium-low.
Cook, stirring constantly, for 5–8 minutes until the mixture thickens and pulls away from the bottom and sides of the pan.
Remove from heat and stir in the butter and vanilla until smooth.
Pour the mixture into a bowl, cover, and chill in the refrigerator for 1½ hours or until firm (or chill overnight).
Place sprinkles or chopped chocolate in a shallow bowl. Lightly rub a small amount of softened butter on your hands.
Scoop rounded teaspoons of the chilled chocolate mixture and roll them into balls, then roll each ball in the sprinkles or chopped chocolate. Place on a baking sheet lined with parchment paper.
Refrigerate the truffles until set. Keep chilled and serve cold.