Melt the chopped dark chocolate using a double boiler or in 30-second intervals in a microwave until smooth; set aside to cool slightly to room temperature.

Ensure your mixing bowl and whisk are completely free of grease or oil residue.

Add the aquafaba and cream of tartar to the mixing bowl and whisk on high speed until stiff peaks form, which typically takes 5 to 8 minutes.

Slowly drizzle in the maple syrup and vanilla extract while continuing to whisk for 1 more minute.

Gently fold one-third of the whipped aquafaba into the melted chocolate to lighten the density.

Carefully fold the chocolate mixture back into the remaining whipped aquafaba using a silicone spatula until no white streaks remain, maintaining as much aeration as possible.

Portion the mousse into individual serving dishes and refrigerate for at least 2 hours to allow the structure to set.