Turn the oven on and set it to 190℃ (375℉, Gas 5). Line a 20 x 30 cm tin with baking paper.
Whisk the eggs, sugar, salt and orange zest together with electric beaters until the mixture has tripled in volume and is thick, pale and almost mouse-like. This takes around 5 minutes on high speed. You can do it by hand, but it will require some elbow grease and take longer.
Fold in the warm water with a large metal spoon, then sieve in the flour and cocoa powder. Fold in carefully, so that you don't loose too much air, until everything is just combined.
Scrape the mixture into the tin, then spread it with a knife until it's filled the corners and is even. Bake in the middle of the oven for 8 minutes, or until well risen and springy to the touch.
Place the tin on a wire rack. Cover with a damp tea towel and leave to cool.
Whilst the sponge is cooling, break the chocolate into a bowl. Add the honey and 25 ml of the orange juice. Place over a pan of hot, but not boiling, water. Leave for a few minutes until the chocolate has mostly melted. Remove from the heat.
Bring the cream to a near boil in a small pan, then pour over the chocolate. Stir from the inside out until everything is melted, smooth and combined. Leave to cool for a few minutes.
Turn the cooled sponge out onto a clean piece of baking paper and peel off the original piece.
Add a teaspoonful or two of the orange juice to the strawberry jam and stir to make it a good spreading consistency. Be careful it doesn't become too runny.
Spread this over the sponge, making sure you leave a small gap around the edges, so that it doesn't all squidge out when rolled.
Using the baking paper to aid you, carefully roll the sponge up lengthways.
Spread the ganache roughly over the roll, so that it resembles tree bark. Leave the ends clear. If you prefer, you can use a fork to roughen the surface up.
Leave for an hour or so for the ganache to set.
The cake should last for 4-5 days if kept in a cool place in a sealed container.
Please note: calories and other nutritional information are per serving. They're approximate and will depend on serving size and exact ingredients used.
WARNING If you feel tempted to eat the ivy, please don't. It's poisonous and is used in the photo for decorative reasons only.