Heat the oven to 180C fan forced, and line a 25cm × 38cm swiss roll tin with baking paper.
To make the sponge, sift the flour, cocoa and cinnamon into a large bowl and leave to one side. Separate the eggs, putting the whites and yolks into separate large clean bowls. Whisk the yolks with 1/2 the sugar for 5 minutes or until thickened and pale. Whisk the egg whites, sprinkling in the remaining sugar with a pinch of salt, until they form soft peaks. Add 1/3 of the whites to the yolk mixture, folding until combined. Finally, add 1/3 of the flour mixture. Repeat the process 2 more times, folding until evenly combined. Add the milk to loosen slightly.
Pour the batter into the tin, and bake for 10-12 minutes or until firm to the touch.
Once baked, turn out the sponge onto a piece of baking paper and peel off the old piece of baking paper - it may crack a bit but don’t worry, you’re going to cover it with ganache. Cool completely.
To make the ganache, put the dark chocolate into a medium bowl. Heat the cream until just simmering, pour over the chopped chocolate and gently stir until fully combined. Leave in the fridge to set for 20 minutes until spreadable.
To make the filling, whip the cream with the Baileys (if using), and icing sugar until just holding soft peaks. Unfold the swiss roll and spread the cream over the cooled sponge, leaving 2cm clear at the shorter ends of the cake. Scatter with chopped hazelnuts, roll the cake into a tight spiral and carefully transfer to a serving plate.
Evenly spread the ganache over the cake and create a tree bark effect using a non-serrated knife. Leave to set in the fridge for 10-15 minutes.
Garnish with whatever you like, we recommend brittle, nuts or berries.