Chocolate Pastry Cream:
Follow the instructions for the pastry cream and keep refrigerated until needed. Make at least the day before.
Prep: Preheat the oven to 350 and place the rack into the lower third. Trace the bottoms of the 6-inch round pans on a piece of parchment and cut out so that they fit inside the pans. Spray the pans with baking spray and place the parchment circles in the pans. Measure and powder the erythritol if not using Sukrin Icing Sugar.
In a smallish bowl, add the two whole eggs, the egg white, buttermilk, vanilla, and stevia glycerite. In another bowl, add the almond and peanut flours, baking powder and soda, salt, and xanthan gum. Mix thoroughly with a whisk until combined.
"Peanut Butter" Cake: In a medium bowl, mix the butter with a hand mixer until soft and smooth. Add the sweetener and beat on medium-high speed until light and fluffy (about 1 1/2 minutes). Add 1 whole egg and beat until completely incorporated and still light and fluffy (1 minute) making sure to scrape down the bowl at least once.
Remove the beaters from the butter mixture and give the liquid ingredients a quick mix. Add half of the dry ingredients into the butter mixture and mix on medium-high speed until fully incorporated. Add half of the liquid ingredients and mix again, making sure to scrape down the bowl completely at least once. Repeat the procedure with the dry and wet ingredients making sure to scrape the bowl with each addition. The mixture should be well beaten, thick and almost mousse-like in texture.
Visually divide the mixture in half and spoon into the cake pans. Lift the cake pans, one at a time, a few inches off of the counter and let them fall, ridding the mixture of large air bubbles. Do this twice each. Then with a small off-set spatula, spread the batter to the edge of the pans and smooth the tops. I left a very slight dome in the center of my pans.
Bake: Place the pans into the oven and turn the heat up to 400F for 8 minutes. Then, turn it back down and bake for another 30-35 minutes or until a toothpick inserted in the middle comes out clean. Cool the cakes completely before assembly.
Peanut Butter Whipped Cream: (make the day of assembly) Add the peanut butter, sweetener and 1/4 cup of whipped cream into medium bowl. Mix until smooth. Gradually add the remaining whipping cream while beating on low. When added, whip the cream until it forms soft peaks. Sprinkle the xanthan gum and whip until stiff. Put 1/3 of the whipped cream into a pastry bag fitted with the large open round tip.
Pastry Cream: remove the chocolate pastry cream from the refrigerator and whip on medium-high until lighter in color and smooth (about 2 minutes).
Cake Assembly: Carefully slice the dome off of the tops of the cakes. Carefully, cut the cakes in half horizontally. Lay one cake layer on top of your cake plate. Pick-up the pastry bag with the peanut butter whipped cream and squeeze a "thick ring" of whipped cream around the inside edge of the cake. This serves as a dam to hold the chocolate pastry cream inside. Now, add 1/4 of the whipped chocolate pastry cream and smooth all-the-way-out to the whipped cream, with a small off-set spatula. Add another layer of cake, checking to make sure the cake is as level as possible. Repeat the process and add the last layer of cake. Put the remaining whipped chocolate pastry cream into a pastry bag fitted with a closed star tip.
Decorate: With a large off-set spatula, smooth the peanut butter whipped cream around the sides and top of the cake. Pipe tall rosettes with the chocolate pastry cream. I piped 8 of the chocolate "rosettes". Then squeeze the peanut butter whipped cream into smaller mounds in-between each of the chocolate rosettes.