To the bowl of a food processor, add the pitted medjool dates, almond flour, cocoa powder, almond butter, vanilla, and salt. Process for 10-15 seconds or until it forms a dough. The dough should be soft but not sticky. If it's sticky, add a bit more almond flour.
Line a 4x7-inch baking pan with parchment paper. Make sure to leave enough paper on the sides to use as handles, making it easier to remove from the pan later. Transfer the dough to the baking pan and press it evenly into a thin layer. Top with another sheet of parchment paper and flatten using a glass. Set aside.
Melt the dark chocolate over a double boiler. Once melted, remove from heat and let cool for about 10 minutes, or until the chocolate is just warm to the touch.
In the meantime, pour the aquafaba into a large mixing bowl. Using an electric hand mixer, whip the aquafaba into stiff peaks. This take might take up to 10 minutes. You want the aquafaba to be quite thick.
Fold the melted chocolate into the aquafaba using a spatula. Be gentle and don't mix too fast. Stop as soon as the chocolate is fully incorporated; otherwise, you might lose the light texture.
Transfer the chocolate mousse to the baking pan over the no-bake brownie and spread it evenly. Next, you can top it with your favorite toppings: chocolate chips, crushed nuts, cacao nibs, etc.
Refrigerate for at least 8 hours, or until the chocolate mousse has hardened. Carefully remove from the pan and cut into 8 small squares (or make bigger ones). If the chocolate mousse is still too soft, freeze it for about 1 hour. It will make it easier to cut.
Serve fresh! These chocolate mousse brownies will keep for up to 3 days in the refrigerator.