Preheat the oven to 325 degrees F. Grease a 9-inch springform pan with non-stick cooking spray and line the bottom with parchment paper.
In a large bowl, combine the almond meal, confectioners’ sugar, flour, cocoa powder and baking powder. In a small bowl, whisk together the egg whites and melted butter. Fold the wet ingredients into the dry ingredients until just combined. Transfer the batter to the prepared springform pan.
Bake for 30-35 minutes, until the cake springs back when gently pressed and the edges have pulled away from the pan. Cool the cake completely in the pan on a wire rack.
Remove the cake from the pan. Use a serrated knife to trim the top of the cake, so it is even. Line the springform pan with plastic wrap and return the cake to the pan.
For the mousse, place the chocolate in a heatproof bowl over a saucepan with 1 inch of simmering water. Stir occasionally until chocolate has melted. Stir in the vanilla extract. Cool to room temperature.
Use an electric mixer to beat the heavy cream until it has stiff peaks. Fold the chocolate into the heavy cream until it is fully combined. Spread the mousse over the cake. Cover with plastic wrap without touching the mousse and chill in the refrigerator until set, about 2 hours.
Let the cake sit at room temperature for 15-20 minutes before serving. Store in the refrigerator.