Break the chocolate bars into small pieces (or use chocolate chips) and melt in a microwave in 30 second intervals, stirring after each time, until melted. It took me 1 1/2 minutes. Set aside to cool completely. If the chocolate is still warm when you add it to the aquafaba, it will seize and become very grainy.
Drain the aquafaba from a can of chickpeas, measure 3/4 cup and add to a large bowl. Add the cream of tartar and beat for about 5 minutes, until stiff peaks form. I use my stand mixer with whisk attachment. Slowly add in the sugar (more or less to desired sweetness, I like 1/2 cup) and vanilla and continue to beat for another few minutes. It should be very fluffy and have very stiff peaks by this point.
Carefully and gently fold the melted, cooled chocolate into the aquafaba mixture, using a spatula in a circular motion until it's incorporated well. Don't over mix - we are just incorporating the two, not roughly mixing as you don't want to lose all the lightness of the aquafaba mixture, this is what makes the mousse light. Continue mixing until there are no white streaks left, and it's a light chocolate color.
Divide the mixture into 4 small ramekins or little dessert glasses. You can enjoy it right away, or chill in the refrigerator until ready to serve. It will keep for about 3 days in the refrigerator.
Top with vegan whipped cream, if desired, and chocolate shavings or mini chocolate chips. Enjoy!
The mousse will keep in the refrigerator for 3-4 days, and can also be frozen.