Preheat the oven to 180C/350F degrees.
Grease and line a rectangle baking tin ( 9 x 13 inch, 22.9cm x 33cm) with parchment paper.
Whisk the egg yolks in a medium bowl until they thicken.
Add the monkfruit sweetener and whisk until thick.
Add the cocoa powder and mix well.
In another medium bowl, whisk the egg whites until stiff peaks form.
Fold the whisked egg whites into the chocolate mixture.
Pour the mixture into the tin, smooth out to all of the edges and bake for 20 minutes, until springy in the centre.
Remove from the oven and allow to cool for 5 minutes.
Put a large piece of parchment paper on your countertop and place the baking tin on the parchment paper, sponge facing down. Remove the tin from the sponge.
Using the parchment paper on the top of the sponge, gently roll up the cake. This is to keep the sponge in the correct shape before adding the filling.
Set aside to cool.