Preheat the oven to 180°C and line a baking tray with greaseproof paper
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Separate the eggs into two bowls and beat the whites to form stiff peaks. Set aside
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Beat the egg yolks, apple cider vinegar and sweetener for a minute. Add the rest of the sponge ingredients and beat till combined.
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Gently fold in the egg whites till fully combined. Adjust sweetness or cocoa to taste.
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Pour the mixture onto the lined baking tray, and spread it evenly using the back of a spatula or spoon. Bake in the oven for 15-20 minutes until cooked through, and allow to cool on a cooling rack.
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While the cake cooks, get on with the frosting and filling. For the frosting, beat butter and mascarpone with a blender or electric beaters for about 2 minutes until smooth.
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Slowly add the sweetener and continue to beat for about 8 minutes until pale, fluffy and the sweetener has dissolved. Add vanilla, mix, and adjust sweetness to taste.
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At this stage, before you add the cacao, remove 80g of frosting and set aside for the cream filling. Add cacao to the frosting and beat till fully combined. Adjust to taste and set aside.
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To make the cream filling, beat the 80g of frosting with 100g of mascarpone until light and fluffy (it wont look like a lot, but you only need a thin layer).
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Once the cake is cool, evenly spread the cream filling on top. Starting from the longest edge, roll the cake.
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At a diagonal, slice about 2 inches off the cake (this will be the branch). Place the log on a serving platter, with place the diagonal side of the branch against the side of the log.
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Using a pallet knife, get creative and and ice the log with the silky, chocolate frosting.‍