Pre-heat the oven to 160ºC.
Spray a 26 x 38 cm Swiss roll tin or shallow baking tray with a little low calorie cooking spray and use to grease the tin.
Line the base and sides of the tin with baking parchment paper.
Place the eggs, sweetener, caster sugar and vanilla essence in a large bowl. Whisk with a hand held electric whisk for about 10 minutes on high speed, until thick, pale and creamy. The mixture needs to be thick enough for a 'ribbon' trail to be left on the surface when the whisk is lifted out.
Sift together the flour and cocoa powder into a medium bowl. Sift once more, into the whisked mixture.
Using a large metal spoon, gently fold the flour and cocoa powder in until just evenly mixed. Be careful not to over mix the mixture otherwise you will knock out the air you have incorporated.
Pour the mixture into the prepared tin and gently spread out using a palette knife.
Place in the oven for 10 - 12 minutes. Gently press the centre of the sponge, if it springs back and leaves no indent then it is ready.
Remove from the oven and turn out, sponge side down, onto a sheet of baking parchment paper.
Carefully peel away the lining paper from the sponge and discard.
Using a round bladed knife, score a line along the narrow end nearest to you, approx 2.5 cm in from the edge.
Whilst the sponge is still warm, and starting from the scored narrow end, carefully roll up into a Swiss roll, using the sheet of baking parchment paper to help you. Make sure the Swiss roll is seam side down and leave to cool completely on the sheet of baking paper.
Place the cocoa powder, white granulated sweetener and water in a small saucepan and stir with a wooden spoon until smooth. Place over a low heat and bring to a gentle simmer.
Simmer gently, for 1 - 2 minutes, stirring, until smooth, glossy and the consistency of melted chocolate. Using a spatula, scrape into a small bowl and leave until just cool.
Place the cream cheese in a medium bowl and mix with a wooden spoon until softened and smooth. Add the cocoa mixture and stir until evenly mixed.
Carefully unroll the cooled Swiss roll on the baking parchment paper. Spread half of the frosting evenly over the sponge, and roll up to form a Swiss roll. Transfer to a serving plate, and place it seam side down.
Spread the remaining frosting all over the Swiss roll, and roughly texture with a small palette knife to resemble bark. Decorate with a sprig of holly, and red berries of your choice. Dust with icing sugar and chill for a minimum of 1 hour before cutting into 10 slices.