Start by making the sponge. Grease a 20cm cake tin with butter and lightly dust with flour. Whisk together the eggs and castor sugar until stiff and pale in colour. Combine together the flour and cocoa powder. Gently fold flour mixture into the egg mixture. Taking care not to over handle the sponge mixture, pour into the prepared cake tin and bake at 175°c for approximately 20 minutes. The sponge cake should be springy when touched by your finger tips and shrinks from the sides. Meanwhile, make the syrup. Bring the water and sugar to the boil in a saucepan. Remove from heat and allow to cool completely before pouring in the alcohol. Next, make the chocolate mousse by melting the chocolate in with half of the cream in a saucepan. Mix the sugar with a spoon of water and cook until it reaches a temp of 121°c. Whisk the yolks until thick and stiff. Carefully pour in the hot syrup, whisking constantly. Continue whisking until the mixture cools down. Pour in the ganache. Whip the remaining cream into soft peaks. Fold the cream into the chocolate mixture. To put the cake together, slice the sponge cake with a sharp serrated knife into 3 slices. Put the first slice of sponge on a cake board, in a 22 cm cake ring. Soak the sponge with the syrup and cover with a thick layer of chocolate mousse. Repeat this process with the remaining sponge and mousse. Put the cake in a refrigerator and allow chilling for a couple of hours before removing from the ring. Decorate cake with chocolate shavings and strawberries.