Preheat the oven to 375 °F. (190 C)
Line your Jelly Roll pan with parchment paper.
In a large mixing bowl, beat the egg whites until soft peaks begin to form.
Add the ½ cup of sweetener and continue beating until stiff peaks form.
Set aside (chill)
Sift the flour, cocoa powder, baking powder and baking soda together into a small bowl.
In a separate mixing bowl, beat the egg yolks and the vanilla extract for about 2 minutes.
Add the butter and beat until well combined.
Add the remaining sweetener and continue beating for 2 more minutes.
Slowly add the flour mix to the egg yolk mixture, beating on low speed, just until smooth.
Fold in the egg white mixture. (using a rubber spatula works the best)
Spread the batter evenly in your jelly roll pan. (an offset spatula works best)
Bake for 12 to 15 minutes, or until the cake bounces back when lightly touched.
While the cake layer is baking, mix 1 tsp cocoa powder with 1 tsp sweetener and sprinkle onto a clean lint free towel.
Remove the cake from the oven and run a knife around the edge of the jelly pan to loosen and cake that is stuck to the sides of the pan.
Invert the cake onto the prepared towel.
Peel off the parchment paper.
Carefully roll the cake in the towel and fold the ends of the towel under.
Place the rolled cake on a wire cooling rack to cool.