To make the ganache icing: Put the finely chopped chocolate into a heatproof bowl and set aside.
Put the double cream into a small saucepan over a medium heat on the hob and heat until it is bubbling on the surface. Remove the cream from the heat and pour over the chocolate in the bowl. Allow to sit for 5 minutes.
Beat the cream and chocolate together until smooth. Leave at room temperature, uncovered, in a cool part of your kitchen.
Make the filling: Melt the white chocolate. You can either heat it in the microwave for 30 seconds, then mix for 30 seconds and repeat until melted (be sure to mix very well every 30 seconds even if it doesn’t look like it needs it). OR you can melt it in a small heatproof bowl over a pan of simmering water (do not let the bowl touch the water!) Set aside for 5 minutes until it’s room temperature.
Once the chocolate is cool, put the cream, cream cheese and chocolate into a large bowl and whisk until it forms stiff peaks. Put it in the fridge while you make the sponge.
Make the sponge: Preheat your oven 180℃ fan/200℃/Gas Mark 6/350F. Line the baking tin with baking parchment or baking paper (not greaseproof paper). Press the paper right into the edges.
Put the egg yolks in a food mixer and whisk for a couple of minutes. Sprinkle over the soft brown sugar, breaking up any lumps, and whisk until you have a thick mixture. Sift in the cocoa and continue to whisk until completely combined.
In a clean bowl, whip up the egg whites to soft peaks. Tip about a third of the egg whites into the chocolate yolk mixture and beat with a spoon until fully combined (this loosens the mixture slightly so it’s easier to fold into the rest of the egg whites).
Now tip that chocolate yolk mixture into the bowl with the remaining whites. Fold everything carefully together with a spoon, trying not to knock the air out. Pour this into the tin and spread out evenly. Bake for 25-30 minutes until the top is dry and bouncy when pressed with your finger. Remove from the oven and set aside to cool for 5 minutes.
Lay a fresh sheet of baking paper on your work surface, larger than the cake, and sprinkle over the caster sugar. Carefully lift the baking paper underneath the cake, so you take it off its baking tray, and flip it over onto the caster sugar. The top of your sponge is now sitting on top of the caster sugar and the bottom of your sponge, covered in the baking paper it cooked on, is facing upwards.
We are now going to roll this up, while it’s still warm. (If it’s too hot to handle, leave it for 5 minutes, but you must roll it while it’s still warm. This avoids it cracking later.) Leave both sheets of baking paper in situ and roll up THE LONG SIDE of the cake, using the baking paper to help you; it doesn’t need to be too tight or too neat. Set aside to cool for 1 hour.
While the sponge is cooling, check on the ganache. Depending on how warm your kitchen is, the setting time can vary a lot. If it is scoopable, spreadable texture now, continue to leave at room temperature. If it’s still quite runny, put it in the fridge while the sponge cools to firm it up.
To assemble: When the sponge is fully cold, carefully unroll it and peel the baking paper off the sides. Gently open out the rolled sponge just enough to be able to add the filling. Spoon on the raspberry jam and smooth it all over the surface.
Spoon the white chocolate cream on top and carefully smooth it out. It is helpful if you leave a gap of about 3cm along one long edge, which will be the end of your roll - this helps it to be less messy when you roll it up!
Roll the sponge back up, complete with the filling, following the same rolling as before. You should have a long, filled log. Now, use a sharp knife to cut about a quarter of the log off, at an angle, and reposition it against the side of the log. Sit the whole thing on a chopping board or covered tray.
Check on your ganache. Give it a good stir to check it is smooth and spreadable (if not, and it’s still too runny, put it back in the fridge for more time until it’s thickened up). Spread or pipe the ganache all over the log. Use a fork to make ‘log’ patterns and decorate with berries and some holly to add to the woodland theme!