Melt the chocolate: Microwave the chopped dark chocolate in 15–20 second bursts, stirring between each, until smooth.
Let it cool for 2–3 minutes so it’s warm but not hot.
Mix the base: In a medium bowl, whisk the Greek yogurt, protein powder, cocoa powder, vanilla, salt, and 2 tablespoons of milk until smooth.
Sweeten to taste: Add your sweetener and whisk again. Adjust sweetness now; the flavor won’t change much after chilling.
Blend in the chocolate: Stream in the melted chocolate while whisking. It will thicken the mixture and add shine.
If the mousse seems too thick, whisk in another splash of milk.
Chill: Spoon into two to three small bowls or glasses. Cover and chill for at least 30 minutes for the best texture.
Finish and serve: Top with berries, shaved chocolate, or nuts right before serving. Enjoy cold.