Heat the oven to 375°F. Coat four 6-ounce ramekins with the softened butter, then add the 2 tablespoons sugar and rotate the ramekins until the bottoms and sides are evenly coated.
Melt the bittersweet chocolate and 2 tablespoons butter together until smooth. Whisk the whole milk and cornstarch together, add them to the chocolate, and cook over low heat for 1 to 2 minutes until slightly thickened. Stir in the vanilla extract and fine salt, then cool for 5 minutes.
Beat the aquafaba and cream of tartar until soft peaks form. Gradually beat in the 3 tablespoons granulated sugar until the foam is glossy and holds firm peaks.
Fold one quarter of the aquafaba foam into the chocolate base to lighten it. Gently fold in the remaining foam in two additions, stopping as soon as no large white streaks remain.
Divide the batter among the prepared ramekins, filling each about three quarters full. Run a clean fingertip around the inside edge of each ramekin to help the souffles rise evenly.
Bake for 13 to 16 minutes until the souffles are tall, set around the edges, and still slightly soft in the center. Serve immediately for the lightest texture.
