Preheat the oven to 350 degrees. Mix together all of the ingredients for the cake.

Once the batter is completely blended, transfer to a well greased 9x13 baking dish in an even layer.

Bake for 20-25 minutes until browned lightly and cooked through.

Let the cake cool to room temperature.

In a heat resistant bowl over a pot of boiling water, heat the egg yolks and ¼ cup of the sugar substitute. Whisk continuously until thickened and foamy.

Remove from the heat, and whisk in the mascarpone.

In a separate mixing bowl, combine the heavy whipping cream and vanilla extract. Whip until medium peaks have formed, and then fold carefully by hand into the mascarpone mixture.

Place the egg whites and ¼ cup of sugar substitute into a mixing bowl, and beat until stiff peaks have formed. Fold the egg white mixture into the cream mixture.

Cut the cake in half lengthwise. Pour some espresso over one half of the cake and allow it to soak in. Place the soaked cake half on a serving tray or plate.

Cover the soaked cake in a thick layer of the cream mixture.

Pour espresso over the other half of the cake, then place it on top of the cream layer.

Smooth another thick layer of the cream mixture on top of the second cake layer.

Sprinkle cocoa powder over the top of the tiramisu, then refrigerate for at least 2 hours before serving.