Preheat the oven to 350F/177C. Line two, 8-inch (20-cm) cake pans with parchment paper; do not grease the sides.
Place the eggs into a stand mixer bowl and whisk on high speed for 7 to 9 minutes, until eggs are pale, thick and voluminous. In a separate bowl, combine the dry ingredients: almond flour, stevia, cocoa powder and baking powder. Sift the dry ingredients into the eggs in small increments, folding gently but thoroughly after each addition.
Add the vanilla, coffee and oil last. Fold the oil into the cake batter briefly, for about 30 seconds. Divide the cake batter evenly between the two pans and use an off-set spatula to spread the batter evenly to the edges. Bake the layers in the preheated oven for 20 to 22 minutes, until the center is set. Allow the cake layers to cool in the pans.
For the Keto Chocolate Mousse:
Meanwhile, prepare the chocolate custard. In a medium-sized bowl, combine the egg yolks, stevia, vanilla and cocoa powder. Whisk for a minute until a smooth paste forms. Begin adding the heavy cream and whisk until the mixture is smooth. In a separate measuring cup, whisk the corn starch into the almond milk, then pour the almond milk into the custard mixture.
Pour the custard mixture into a sauce pan and cook over medium-low heat, stirring constantly with a rubber spatula, for 4 to 5 minutes. The custard needs to thicken and hold its shape on the back of a spoon. Remove the custard from heat and add the butter. Whisk until the butter is completely melted, then cover the custard and place into the refrigerator to cool completely.
Making the Whipped Cream:
Once the custard and the cake layers are completely cooled, prepare the mousse. Pour the 2 cups (475 ml) chilled heavy cream into a mixer bowl and whisk on high speed for 3 to 4 minutes until stiff peaks form. In a small ramekin, combine the water and gelatin and whisk until smooth. Microwave the gelatin mixture for 30 to 45 seconds, stirring occasionally, until the gelatin is completely dissolved. Do not wait for the gelatin to cool; add it immediately into the whipped cream; whisk on high speed for 30 seconds.
Add about 1 cup of the whipped cream into the cooled custard and fold gently. Then, add the custard to the whipped cream. Fold gently but thoroughly, until the mixture is smooth and even.
Assembling the Mousse Cake:
To assemble the cake, place one cake layer into the bottom of an 8-inch spring form pan lined with plastic or an acetate cake collar. Add half of the chocolate mousse and spread it evenly to the edges, then top with the second cake layer. Add the remaining chocolate mousse on top and spread evenly with an off-set spatula. Refrigerate the mousse cake for at least 6 hours, or overnight.
For the keto whipped cream, combine the cream, stevia and vanilla in a mixer bowl. Whisk on high speed for 3 to 4 minutes until stiff peaks form. Transfer the whipped cream into a pastry bag tipped with a star tip; I used Ateco tip #847.
Release the spring form pan and transfer the cake onto a stand. Garnish with a drizzle of melted dark chocolate and keto whipped cream. Keep cake refrigerated until ready to serve.