Preheat oven to 350F. Grease pan and line with parchment paper, cutting into the corners to fit. Place chocolate in a small saucepan with 1/4 cup of water and stir over low heat until the mixture is smooth. Set aside to cool slightly.
Using a electric mixture whisk the egg yolks and sugar in a large bowl until thick and pale then whisk in the chocolate mixture. Completely clean and dry the electric whisk attachment and whisk the egg whites in a clean dry bowl until soft peaks form. Using a large spatula, fold the egg whites into the chocolate mixture 1/3 at a time until just combined. Pour into the baking sheet and smooth the surface. Bake for 18-20 minutes.
In the mean time, strain out your cherries, reserving the liquid and cut in half, set aside. Place the cherry liquid, sugar and rum on a medium heat to reduce, roughly 5 minutes. Set aside.
Place a piece of parchment paper slightly larger that the cake onto your surface and sift the cocoa over top. Turn out the cooked cake onto the cocoa. Peel away the paper. Carefully roll up the cake using the paper as a guide. Set aside and let cool completely.
Whisk up the whipping cream to soft peaks just before assembly. To assemble, unroll the cake and brush with the syrup, spread evenly with the cream, place the cherries around evenly and then sprinkle some chocolate shavings. Re-roll the cake and top with more whipping cream, cherries and chocolate.