Preheat the oven to 180C/170C Fan/Gas 4. Grease and line two 20cm/8in sandwich tins. Set up a freestanding mixer with a paddle attachment.
For the cake, place all of the ingredients, except the boiling water and icing sugar, into a mixing bowl. Beat until all the ingredients are incorporated and the mixture is smooth. Slowly add the boiling water and mix (the mixture will be very runny).
Divide the batter equally between the tins and bake for 25-30 minutes.
When the cakes are cooked, let them cool in the tin for 10 minutes. Line a wire rack with greaseproof paper and dust it with icing sugar (the cakes are sticky). Turn out the cakes and leave to cool completely on the prepared rack. When the cakes have cooled, cut both the cakes in half horizontally, to make to 4 layers.
For the cherry jam, pour the cherries, sugar and lemon juice into a saucepan. Heat gently until the sugar is dissolved. Put 4 tablespoons of the liquid from the pan into a mug and mix in the cornflour. Stir the mixture into jam, along with the cherry and chocolate flavour syrup and cherry essence. Bring to the boil, stirring constantly until thick. Reduce the heat to a simmer. Pour the jam onto a large plate or shallow bowl and transfer to the fridge to cool. (CAUTION: Be careful not to burn yourself when handling the jam.)
For the chocolate mirror glaze, mix the gelatine with 3 tablespoons water and set aside to dissolve. In a saucepan, heat the sugar, cream and 6 tablespoons water until it boils. Remove from the heat and add the glucose and cocoa powder. Stir in the gelatine, mix well and strain through a fine sieve. Set aside to cool to room temperature.
For the syrup, mix the ingredients together and set aside.
For the chocolate buttercream, melt the chocolate. You can do this in a microwave or in a heatproof bowl suspended over a bowl of simmering water. Set aside to cool slightly (the chocolate should be warm but not hot).
Cream the butter and icing sugar together in a large bowl. When the mixture is smooth, add the melted chocolate and mix until combined. Add a little milk if the buttercream is too thick.
For the chocolate ganache, place the chocolate in a heatproof bowl. Gently heat the cream in a saucepan until it just comes to the boil, then pour it over the chocolate. Stir the mixture until the chocolate is melted. Set aside in a cool place (the ganache will have a spreadable consistency when it’s cool).
For the whipped cream, beat the cream using a hand held mixer until soft peaks form when the whisk is removed. Add the icing sugar, cornflour and milk powder and beat until firm. Refrigerate until ready to use.
For the chocolate decorations (if using), melt two-thirds of the chocolate in a heatproof bowl suspended over a pan of simmering water. When the chocolate is melted, remove it from the heat. Add the remainder of the chocolate and stir until melted. Cover the bowl with a tea towel to retain some of the heat.
Layout the acetate and drizzle the melted chocolate into decorative shapes. (For a neater finish, use a piping bag.) Set aside in a cool place for the shapes to set.
To assemble the cake, brush each layer of cake with the syrup. Place a layer of cake on a serving plate. Spread a layer of chocolate ganache on top and then a layer of whipped cream. Spoon over some cherry jam. Repeat with two more layers of cake and place the final layer on top.
Cover the whole cake in butter cream (if you have time freeze the cake for 10 minutes to help it set). Pour the chocolate mirror glaze over the top of the cake (you should have a smooth glossy surface on the top of the cake with the glaze drizzling down the sides attractively).
Decorate the cake by piping rosettes of the chocolate ganache on top, add the chocolate decorations (if using) and finish with a few glacé cherries.