Melt the chocolate: Place the chopped dark chocolate in a microwave-safe bowl and heat in 20–30 second bursts, stirring between each, until smooth.
Set aside to cool for 2–3 minutes so it’s warm but not hot.
Blend the base: In a blender or food processor, add Greek yogurt, chocolate protein powder, cocoa powder, milk (start with 1/3 cup), vanilla, sweetener, and a pinch of salt. Blend until silky, scraping down the sides as needed. The texture should be thick but pourable.
Add melted chocolate: With the blender running on low, stream in the melted chocolate.
Blend just until fully incorporated. This step adds body and a glossy finish.
Adjust consistency and sweetness: If the mousse is too thick, add another splash of milk. Taste and adjust sweetener.
If it tastes flat, a tiny extra pinch of salt brightens the chocolate.
Optional flavor boosts: Pulse in 1–2 teaspoons espresso powder for depth, a swipe of orange zest for brightness, or 2 tablespoons nut butter for extra richness.
Portion and chill: Spoon the mousse into 4–6 small cups or jars. Cover and refrigerate for at least 1 hour, ideally 2–4 hours, to set and develop flavor.
Garnish and serve: Top with shaved chocolate, berries, or cocoa nibs right before serving. Enjoy cold.