In a double boiler, melt the chocolate. Then set it aside and let it cool off.
Whisk (using a stand mixer or electric hand mixer) the aquafaba until it has stiff peaks. Add the guar gum and whisk further. This should take about 7 minutes. This is the most important step, don't rush this.
Add the cooled-off melted chocolate to the whisked aquafaba and carefully fold (don't mix) it in. The aquafaba will reduce in size, this is normal.
Divide the chocolate mousse into jars (this works best with an ice cream scoop). Then place them in the fridge for at least 8 hours or overnight.
Then it's ready to serve. You can add shaved chocolate, whipped cream, fruits or nut butter on top!
Notes
*alternatives for guar gum: locust bean powder (1/2 teaspoon), cream of tartar (1/2 teaspoon)Tips
Whisk the aquafaba long enough. It should have stiff peaks. This will take at least 7 minutes.
Let the melted chocolate cool off before adding it to the whisked aquafaba.
Let the chocolate mousse set in the fridge for at least 8 hours or overnight.
You can keep it in the fridge for up to 3 days. This also means you can prepare it ahead.