Make the ganache first as it needs time to cool. Heat the double cream and butter in a pan on a low heat, you want to to be warm but not so hot you couldn't hold your finger in it
Take it off the heat and add the dark chocolate broken into pieces. Leave for a few minutes, then stir and the chocolate should have melted. You can put back on a very low heat if needed to melt all of the chocolate. Put aside to cool and thicken, or in the fridge if you're a rush
Pre-heat your oven to 160C Fan/180C/400F/Gas Mark 6, line a swiss roll tin or a baking tray with a lip, with baking paper. The one I used was a Wilton tray that is 33.7 x 23.5 cm (13.25 x 9.25 inches)
For the sponge, whisk the eggs and sugar together with an electric whisk or in a food mixer for around 8 minutes or until it reaches 'ribbon stage' - this is when you can drizzle the mixture in a figure 8 and it takes more than 3 seconds to sink back in
Sieve the plain flour and cocoa powder into a bowl and then fold it into the egg mixture gently, trying not to lose the volume
Pour the batter into the lined tray and gently spread the mixture out into each corner
Bake it for 12 minutes. Immediately tip it out onto a clean sheet of baking paper dusted with icing sugar and roll it up, rolling the clean sheet of baking paper with it. Leave it to cool fully
To make the chestnut puree (if you are not using a ready made one) put the cooked and peeled chestnuts, icing sugar, water and vanilla into a blender and whizz up into a puree
Whip the double cream and stir the chestnut puree into it
Unroll the sponge and spread the caramel filling all over it evenly, then spread the chestnut cream over the caramel sauce, then roll the sponge back up carefully and place onto your serving plate
Spread the chocolate ganache all over the roll. You want this to be quite rough as the roll is meant to look like a log. If the ganache is too solid from the fridge you can heat it up in the microwave to soften it, do 10 seconds to start with, then stir and do more very short blasts if needed as you don't want it to get too runny
Bash up 1 flake bar and sprinkled it all over the wet icing
Drizzle over the salted caramel sauce and top with the salted caramel fudge
Serve immediately, store any leftovers in the fridge for up to 2 days