Melt the chocolate. Chop the vegan chocolate into small pieces. Place it in a microwave-safe bowl with the coconut oil. Microwave in 15–20 second intervals, stirring well between each interval, until completely smooth. Set aside to cool to approximately 95°F (35°C). The chocolate should remain fluid but should not feel hot.

Whip the aquafaba. Add the aquafaba and cream of tartar to the clean bowl of a stand mixer or a large mixing bowl. Whip on high speed for about 4–6 minutes, or until stiff peaks form and the foam holds its shape.

Sweeten and flavor. With the mixer running, gradually add the sugar a little at a time. Continue whipping until glossy and medium-stiff to stiff peaks remain. Add the vanilla paste and sea salt and mix only until incorporated.

Lighten the chocolate. Fold approximately one-third of the whipped aquafaba into the cooled melted chocolate. This first addition lightens the chocolate mixture and makes it easier to incorporate the remaining foam.

Fold together. Gently fold the remaining aquafaba into the chocolate mixture in additions. Cut through the center with a rubber spatula, sweep along the bottom of the bowl, and fold the mixture over itself. Rotate the bowl as you work. Stop as soon as no large white streaks remain. Do not overmix or the mousse will lose volume.

Portion and chill. Spoon the mousse into individual glasses or transfer it to a piping bag or covered container. Refrigerate for at least 30–45 minutes. For a firmer texture, chill for up to 3 hours.

Serve. Serve chilled. Garnish, if desired, with fresh berries, shaved vegan chocolate, cocoa powder, flaky sea salt, or another vegan garnish of your choice.