Bake chocolate sponge cake: Preheat the oven to 400°F with a rack in the upper third. Spray a 17-by-12 inch rimmed baking sheet with pan spray and line the bottom with parchment paper.
To make the cake, stir together the flour, cocoa, and salt in a small bowl.
Place the Truvia® natural sweetener into a measuring cup.
Put the whole eggs into a mixing bowl. Using an electric mixer, beat the eggs with the whisk attachment at high speed for several minutes, until thick, pale, and yellow. Slowly pour in the Truvia® natural sweetener in a slow and steady stream. Continue to whip the eggs for about 3 minutes. Add flour mixture into the eggs, and mix on low speed, just until incorporated. Increase speed to medium and mix well for 10-15 seconds.
In a clean bowl using clean beaters, beat the 4 egg whites at high speed until frothy. Add the cream of tartar, then continue to beat until the whites hold a medium peak when you lift the beaters, about 2 minutes. A medium peak will look like a soft ice cream curl, it will arch and then curl back over.
Use a large spatula, gently fold the whites into the cake batter until blended and no longer streaky.
Transfer the batter to the prepared pan and smooth out the batter to make sure it's as level as possible in the pan.
Bake just until the cake feels soft and springy in the center when you press it lightly with your fingers, 6-8 minutes. Allow the cake to cool completely, about 30 minutes.
Run a small paring knife around the perimeter of the pan to loosen the cake. Lay out a sheet of waxed paper slightly larger than the pan on a flat surface. Invert the pan over the waxed paper. Remove the pan and carefully peel the parchment paper off the bottom of the cake.
Note: You will need an 8” round spring form cake pan to make the cake. If you do not have this, you can also layer the cake into an 8” trifle mold or glass bowl. To build the mousse cake, cut the cake into two 8” round pieces. Reserve the additional cake scraps for the center layer.
Make coffee syrup: To make the syrup, combine all of the coffee syrup ingredients in a small bowl. Whisk together and set aside.
Make tiramisu mousse: Place ¼ cup of the evaporated milk into a small pot. Whisk in the gelatin and allow to sit for 3 minutes to soften. Place the pot on the stove over medium heat and cook, while whisking, until hot and the gelatin has melted. Do not bring the mixture to a boil. Transfer the hot milk to a mixing bowl, add in the other ¼ cup of evaporated milk. Place this into the freezer for 10 minutes to chill before beating.
In the meantime, combine the Truvia® natural sweetener and cream cheese. Beat using an electric mixer on medium speed for 1-2 minutes. Add in the ricotta and beat for 1 minute.
After 10 minutes, remove the evaporated milk from the freezer. The edges of the milk should be starting to set. Beat the milk for 4-5 minutes with an electric mixer on high speed. It should now resemble whipped cream.
Spoon the cream cheese mixture into the evaporated milk, and mix for 1-2 minutes to combine.
Assemble cake: To assemble the cake, place the first cake circle into the bottom of the cake pan, use a pastry brush, or, spoon ⅓ of the coffee syrup onto the cake. Spoon ⅓ of the tiramisu filling on top of the cake sponge and sprinkle half of the cocoa nibs on top.
Layer the cake scraps in the center layer, neatly creating a circle that will cover the tiramisu filling beneath. Cover the cake sponge with ⅓ of the coffee syrup, top with ⅓ of the tiramisu filling, and the other half of the cocoa nibs. Top with the final circle of cake. Cover the cake with the remaining coffee syrup. Top with the remaining mousse, cover with plastic wrap and refrigerate for at least 1 hour, up to overnight. To serve, dust the top of the cake with the cocoa powder. Sprinkle with the remaining Tbsp of cocoa nibs on top.
If you are using a spring form pan, wrap a hot towel around the outside of the cake pan before you unlatch the sides of the pan. This will warm the pan and help it to smoothly pull away from the sides of the cake. Slice and serve.