Preheat the oven to 180C/160C Fan/Gas 4. Lightly grease two 23cm/9in cake tins and line the base with greased non-stick baking parchment.
Break the eggs into a mixing bowl, add the sugar and whisk until the mixture is pale and thick enough to leave a trail when the whisk is lifted out of the bowl. Carefully fold in the sifted flour and cocoa powder. Turn the mixture into the prepared tins.
Bake for 25-30 minutes, until the sponges are well risen and beginning to shrink away from the sides of the tin. Turn out onto a wire rack to cool and peel off the parchment.
For the filling, drain the jars of cherries, reserving the juice. Chop the cherries roughly into quarters. Measure the cornflour into a small saucepan, make a paste with a small amount of the reserved juice, then stir in the remaining juice. Place over a medium heat and bring slowly to the boil, stirring until thickened, then simmer for 2 minutes. Remove from the heat, add the cherries and leave to cool. Refrigerate until quite firm.
For the decoration, melt the white and plain chocolate in 2 separate bowls set over 2 pans of gently simmering water. Dip 12 of the fresh cherries into the plain chocolate and leave to set on a sheet of non-stick baking parchment. Dip the remaining 12 fresh cherries into the white chocolate and leave to set. Spoon the remaining melted chocolate into 2 separate disposable piping bags.
Draw a credit card size rectangle on a piece of paper and position it vertically. Draw a tree within the rectangle to use as a template. Lay a sheet of non-stick parchment paper over the template. Snip off the tip of the plain chocolate piping bag (start with a very small tip). Pipe 12 (plus a few spare in case they break) chocolate tree shapes onto the non-stick parchment. Leave to harden and set.
When the chocolate on the cherries has set, snip the end of the white chocolate piping bag and pipe thin lines of white chocolate over the plain chocolate dipped cherries. Pipe thin lines of plain chocolate over the white chocolate dipped cherries. Leave to set.
Whip the cream to soft peaks. Divide the cream in half, setting aside one half for the sides and top of the cake.
Cut each sponge into 2 layers using a long sharp knife. Divide the cherry mixture into thirds. Spread one third of the cherry mix over the sliced sponge. Divide one half of the cream into thirds and spread one third over the cherry mix. Top with another sponge layer and continue until you have three layers of cherries and cream. Top with the final sponge layer.
Spread half of the reserved whipped cream around the sides of the cake and cover with the chocolate trees. With the remaining reserved cream, pipe rosettes of cream around the top edge of the cake and decorate with the chocolate dipped cherries.