Add the cold heavy whipping cream, unsweetened cocoa powder, powdered erythritol, vanilla extract, and sea salt to a large mixing bowl.
Using an electric hand mixer, beat on low speed for 15 to 20 seconds just to combine the cocoa powder into the cream without it flying everywhere. Then increase to high speed.
Beat on high for about 60 to 90 seconds, until the mixture thickens into a rich, fluffy mousse with soft peaks. Be careful not to over-beat or it will turn grainy and butter-like. Stop as soon as the mousse holds its shape on the beaters.
Taste and adjust sweetness if needed. Divide the mousse between 2 serving glasses or small bowls.
Top with a dollop of sugar-free whipped cream and scatter chocolate shavings over each. Serve immediately for a light, airy texture, or refrigerate for 30 minutes for a denser, more set mousse.
