Melt the chocolate chips and coconut oil in the microwave in 30-second increments until completely melted, stirring in between increments. Set aside to cool.
Open a 19 oz can of chickpeas and drain in a strain over a bowl or measuring cup to collect 3/4 of the liquid, or aquafaba.
Using a stand or electric hand-held mixer with the whisk attachment, whisk the aquafaba in a large bowl on high until stiff peaks form. This may take up to 10 minutes depending on your mixer. It's ready when it will not slide off a spoon if flipped upside down. Unlike with eggs, you cannot over whisk aquafaba, so don't worry if it's taking a while.
Once stiff peaks have formed, add in the lemon juice and powdered sugar. Continuing whisking until evenly combined.
Once the chocolate has cooled to room temperature, slowly transfer to the large bowl with aquafaba. Whisk on low until fully combined. It’s important for the chocolate and aquafaba to be at the same temperature so as to prevent the chocolate from seizing.
Once all of the chocolate has been whisked into the mousse, transfer the mousse to 4 serving containers and place in the fridge for at least 4 hours to set but preferably overnight.
Serve your mousse as is or with shaved chocolate or a dusting of cocoa powder, whipped cream and fresh berries.