Prepare the coconut cream. Open the chilled can without shaking it. Scoop the firm coconut cream from the top into a chilled mixing bowl, leaving the watery liquid behind.
Whip the base. Beat the coconut cream with an electric mixer until smooth and fluffy. Add the unsweetened cocoa powder, Dutch-process cocoa, powdered sugar, vanilla, and sea salt. Beat just until smooth and evenly combined.
Whip the aquafaba. Add the aquafaba and cream of tartar to a separate clean, grease-free bowl. Beat with an electric mixer until firm peaks form and hold their shape when the beaters are lifted.
Fold together. Fold the whipped aquafaba into the chocolate mixture in 2–3 additions using a spatula. Work gently and stop as soon as no obvious white streaks remain.
Chill. Divide the mousse among 4 small dessert glasses or ramekins. Refrigerate for at least 2 hours, or until chilled and softly set.
Serve. Serve the mousse chilled with your favorite toppings.