Preheat your oven to gas 4 / 180C / 350°F and line a shallow tin such as for brownies or swiss roll with greaseproof paper
Mix the flour, cocoa and salt together
In a separate bowl combine the sugar, water, oil and avocado. Mix well and add to the dry ingredients and combine well
In a small bowl combine the baking powder, soda, vinegar then vanilla. Mix until well frothed up then add to the cake batter. Don't overmix or you'll knock all the air out
Quickly pour into the shallow cake tin and put in the oven for 12-15 minutes or until JUST ALMOST done or you won't be able to roll it
Lay another piece of greaseproof paper on top of the sponge. Use the bottom paper to roll the sponge over jelly/swiss roll style so the top piece of paper is rolled inside. Don't roll too tight or the cake will crack when you undo it. Let cool 15 minutes then unroll carefully - you are not rolling it completely flat, that will break it. Just loosening it
Make up buttercream icing by creaming the butter, sugar and cocoa (you might like more cocoa in it) together, remove the paper and spread on top of the sponge, then roll back up again. Cover the whole sponge including ends in the rest of the buttercream
Drag a fork long ways across the 'log' to make the bark of the wood effect. Don't forget the ends, Do those grains circle fashion. Put the cake in the fridge to harden the buttercream
To make the ganache for the top, first chop up the chocolate quite small. Bring the cream to the boil in a saucepan and as soon as it does, take the pan off the heat and whisk in the chocolate. At this stage you could add a tsp of rum, orange essence or any other flavouring but it's not necessary
Let the ganache cool to the point where it's not hot enough to melt the buttercream icing but is still spreadable and put into a piping bag
Take the log out of the fridge and pipe the ganache all over remembering the ends. It should settle into the grooves you already made nicely but you can gently go over them again if you want to. Put back into the fridge to harden
When ready to serve give a dusting of powdered sugar. Traditionally these are decorated to resemble forests with lots of greenery on top
