Preheat your oven to 220ºC/200ºC Fan and line a 23cmx33cm swiss roll tin with parchment paper (I find its easier to grease the tin so the paper sticks), then leave to the side.
In a large heatproof bowl over a pan of simmering water (Double Boiler), melt the chopped dark chocolate with the 4tbsp cold water - stir occasionally whilst melting, then once fully melted leave to cool. (Alternatively, melt the chocolate in a bowl in the microwave on short bursts, then add the water and beat until smooth!)
Separate the eggs into two different bowls, yolks in one, whites in the other.
Add the caster sugar to the egg yolks and with an electric whisk, whisk until light and creamy (this may take 2-3 minutes).
Once mixed, fold through the cooled dark chocolate.
In the other large bowl, whip up the egg whites with a clean whisk until they form stiff peaks. (Don’t over beat them however as they can go dry and then deflate - they will be unusable then!)
Mix in two spoonfuls of the egg whites into the chocolate and yolk mix so that it loosens the mixture, then carefully fold through the rest of the egg whites as carefully as possible .
Once combined, carefully pour evenly into the prepared tin, and bake in the oven for 12-14 minutes. It won't need any longer as it may start to catch!
Once baked, get a clean tea towel and dust the tea towel with icing sugar.
Flip the cake over onto the tea towe,l so the top of the cake is face down onto the icing sugar covered towel and peel off the parchment paper.
Roll the cake from the longer side, to the longer side (including the towel) so that the rolled cake is longer and thinner, not short and fat! Doing this means it will be far easier to roll again later & decorate.
Leave the cake to cool fully in the towel sausage!
Chocolate Frosting
Melt the dark chocolate and leave to cool slightly.
Beat the unsalted butter & icing sugar together until smooth, then add the vanilla bean extract, and fold through the cooled melted chocolate.
CAREFULLY unroll the cake again and spread a couple of spoonfuls of the frosting onto the cake, and roll back up - it doesn't matter if it cracks as this will be covered up!
At an angle, cut off a few inches of the end of the cake and sit it next to the rest of the cake (like shown in the pictures!)
Cover the cake with the rest of the frosting and create lines using a fork or a skewer, so it looks like bark on a tree!
Decorating Options
I used some green candy melts, melted them down and piped them on, and topped with three red M&Ms! Dust the cake with icing sugar and serve!
Notes
The cake recipe is inspired by Paul Hollywood, the Frosting is my own!
I find it easier to roll it when the cake is warm, but be careful not to burn yourself. If you roll the cake when its cold you might find its much harder and is more likely to crack. However, even if it cracks, you can easily just glue it back together with some of the frosting to form the log shape, and then still decorate as you won't see the cracks once its decorated!
I know the idea of putting water into the chocolate is a little terrifying - but it makes a delightfully thick ganache like chocolate. Just be careful with it - don't over mix the chocolate if its melting over a double boiler, but if you're doing it in the microwave and then adding the water after, beat like crazy so that its smooth!
This cake will last for 3 days in an airtight container, or you can freeze it for up to 3 months!