Preheat the oven to 160ºC. Line a baking tray with baking paper - you may want to spray the tray first with low calorie cooking spray to help the paper stay in place.
Make the cake by placing the eggs, sugar and sweetener into a large mixing bowl. Mix with an electric hand whisk until the mixture is thick, light and has tripled in size.
This will take around 8 minutes. You should be able to see visible ribbons on the top of the mixture when you lift the whisks out.
Sift the flour and cocoa powder into the mixture and add the vanilla.
Gently fold using a spatula until all the flour and cocoa are fully incorporated - you don't want any pockets of flour in the mix but try not to over mix as this will knock the air out.
Pour the cake mixture into the lined baking tray and gently spread out to the corners, making sure the top is level. There's no fat in the sponge so it won't self level in the oven.
Bake in the oven for around 13 minutes, until the top has just started to colour. You will know the sponge is cooked if you gently put your finger on the top and no indent is left. If your finger leaves a mark, pop it back in the oven for another 2 minutes.
Remove from the oven and turn out from the tin onto a clean tea towel. Remove the baking paper carefully.
You now need to roll the cake up in the tea towel to cool - do this by gently rolling the long side of the cake, with the tea towel into a swiss roll. Leave to cool completely.
Meanwhile, mix all of the ingredients for the filling and set aside.
Make the coffee drizzle by mixing the coffee, boiling water and sweetener in a small cup and leave to cool.
Once the cake is completely cooled, carefully unroll from the tea towel. Sprinkle over the coffee mixture - this will soak into the cake - then spread over the mascarpone filling.
Roll back up, cut into 12 slices, and serve! You could dust over a little cocoa too, but this is optional.