Place the chopped sugar-free dark chocolate in a heatproof bowl.
In a small saucepan, heat the heavy cream, butter, and instant espresso powder over medium heat, stirring until the butter melts and the espresso dissolves. Bring just to a gentle simmer, then immediately pour over the chopped chocolate.
Let sit for 1 minute, then stir slowly from the center outward until the chocolate is completely melted and the ganache is smooth and glossy. Stir in the vanilla extract and sea salt.
Cover the bowl and refrigerate for at least 2 hours until the ganache is firm enough to scoop.
Mix the cocoa powder and powdered erythritol together in a shallow dish. Use a small spoon or melon baller to scoop teaspoon-sized portions of ganache, roll quickly into balls with your hands, then roll in the cocoa mixture to coat.
Place the finished truffles on a parchment-lined plate and refrigerate for 30 minutes to set. Store in the refrigerator in an airtight container.
