Preheat the oven to 350° F. Line either a 10-inch by 15-inch or a 12-inch by 17-inch jelly roll pan with baking paper.

Place the carob powder and softened butter into a small bowl. Pour in the boiling water and mix with a fork until smooth. Stir in the vanilla.

Use an electric mixer to whip the egg yolks and sweetener until very light and fluffy, 7 to 10 minutes. Stir in the carob mixture and transfer the new mixture to a medium-size mixing bowl.

If you have only one whip and one mixing bowl for your mixer, wash and dry them quickly and thoroughly before proceeding.

Whip the egg whites and sweetener until stiff peaks are formed. Alternately fold the whites and the cake flour into the carob, egg yolk mixture. It is important that you fold both gently and thoroughly without deflating either the yolks or the whites. Spread the cake batter evenly into the prepared jelly roll pan.

Bake the cake in the preheated oven until it springs back when touched lightly in the center, 15 minutes for the smaller pan, or 18 minutes for the larger pan.

Remove the cake from the oven and immediately cover it with a damp cloth to keep the cake moist. Let the cake cool for about 10 minutes before removing it from the pan. Place a sheet of baking paper on your work surface. Remove the dampened towel and loosen the edges of the cake with a sharp knife. Invert the cake onto the baking paper. Gently peel away the baking paper from the bottom of the cake (now the top of the cake).

Immediately spread on a very thin layer of jam glaze and 1-1/3 cups of filling for the smaller cake, 2 cups of filling for the larger cake. With the point of a sharp knife, lightly score the cake 1/2 inch in from the edge closest to you. Then with the support of the baking paper roll the cake up and place it seam-side down, either on a rectangular piece of cardboard or directly on a serving platter.