Add 1/2 cup of aquafaba (bean water) and 2 teaspoons of lemon juice to a mixing bowl and beat on high for 8 minutes. (A stand mixer is best but you can use a hand mixer.)
After it has mixed for 8 minutes and become very fluffy, add 1 teaspoon of vanilla and 1 1/3 cups powdered sugar and beat again on high for 1 minute.
Add a box of silken tofu to a food processor and plus a few times.
Heat 1 cup of chocolate chips plus 3 tablespoons of butter in a small bowl in the microwave for 1 1/2 minutes. Stir the melted chocolate and butter until combined and then add it to the tofu. (You may need another 30 seconds in the microwave.) (Use a double boiler if you don't have a microwave.)
Blend the silken tofu and melted chocolate in the food processor until very creamy and completely mixed. (You may have to stop and scrape the edges a few times.)
Add the chocolate mixture to your aquafaba mixture and blend again on low for about 30 seconds until combined. (You don't want to over mix or you will lose some of the fluffiness.)
Immediately pour into little bowls, glasses, or ramekins and then place in the fridge for at least 1 hour to set.
Top with vegan whipped cream, chocolate shavings, or berries like fresh raspberries, blueberries, or strawberries just before serving.
Notes
You must use an acid like lemon juice or cream of tartar to make the aquafaba protein bond together and hold its shape.
If your aquafaba is clear and as runny as water, you will need to simmer it on the stovetop for about 10 minutes to make it more concentrated before using it. You can find detailed instructions about how to do that in this post about aquafaba.
Once you add the tofu and chocolate mixture with the aquafaba mixture be sure to only mix until combined. Over-mixing at this point will deflate your aquafaba and make the mousse less fluffy.