Start this recipe by making up the blackcurrant jelly (as directions on the packet). Take 150g of the berries and divide them equally between 8 serving glasses. Pour the blackcurrant jelly equally into the glasses, over the berries, and place them into the fridge for 2 hours to set the jelly. Meanwhile, you can slice up the chocolate cake into 1cm slices. Take the set jelly glasses out of the fridge and cover the top of the jelly with 2 slices of the chocolate cake. Sprinkle the sherry over the chocolate cake, and then place the remaining berries on top. Next, you can prepare the chocolate mousse by putting the cream and chocolate in a saucepan. Heat them together over a low heat till the chocolate has melted, stirring constantly. Pour this chocolate sauce into a bowl and leave to cool for 15 minutes, and then stir in the egg yolk. In another bowl, whip together the caster sugar and the egg white to light and fluffy foam. Gently fold this mixture into the cooled chocolate sauce, till you have a nice chocolate mousse. Using a piping bag or large spoon, carefully fill the glasses equally with the chocolate mousse, covering the berries, cake and jelly. Put the glasses back into the fridge to chill for another hour for the mouse to set. When ready to serve, decorate the top of the chocolate and berry trifles with a few berries.