While the crust is baking, place the chocolate and butter together in a small bowl and microwave in 10 second increments, stirring often. When the chocolate is mostly melted, remove the bowl from the microwave and keep stirring until all remaining chocolate fully melts. Set aside to cool.
Using a stand mixer or hand mixer, add the reduced aquafaba to a large bowl and whisk for 2 - 3 minutes on the highest setting until very foamy.
Add in the sugar and keep whisking until it has fully dissolved into the aquafaba. This may take another 2 - 3 minutes. The mixture will be fluffy and resemble marshmallow fluff when done. Scrape the sides of the bowl down to make sure all sugar is fully incorporated.
Add in the melted butter and chocolate, and vanilla extract and whisk on medium speed to combine.
Place a fine mesh strainer over a medium bowl and add the flour, cocoa powder, and salt. Sift the dry ingredients through the strainer and into the bowl. Press down any large remaining clumps of flour or cocoa powder until all dry ingredients are fully sifted through the strainer.
Add all of the dry ingredients to the wet mixture and whisk together on the low setting. When the ingredients appear to be whisked together fully, remove the whisk from the bowl and use a rubber spatula to fold in any remaining batter on the bottom that may not have mixed fully.
Reduce the oven temperature to 325°F.