This grand and impressive cake is ideal for a celebration.
Preheat the oven to 180°C/160°C fan/350°F/Gas 4.
Make the sponge. Beat together the margarine and sugar in the bowl of a stand mixer fitted with the beater attachment until light and fluffy. Gradually add the beaten eggs and beat until smooth, adding a little cocoa powder or flour if the mixture curdles. Sift the cocoa and flour together, then fold into the batter with the buttermilk and coffee until fully combined. Pour equal amounts of the batter into the prepared cake tins.
Bake the sponges in the oven for 35–40 minutes, or until the tops are springy to touch and a skewer inserted into the centres comes out clean. Remove the sponges from the oven, leave to cool in the tins for 5 minutes, then turn out onto a wire rack to cool completely.
Make the strawberry jam. Put the strawberries, sugar and lemon juice in a frying pan with a little water and place over a low heat for 5 minutes. Increase the heat and boil for 10 minutes, then pour the jam into a shallow bowl and leave to cool.
Make the chocolate ganache. Gently heat the double cream in a pan over a low heat until hot but not boiling. Add the chocolate, take the pan off the heat and leave the mixture for 5 minutes to melt the chocolate. Add the strawberry liqueur and stir until smooth. Transfer the ganache to a bowl and place in the fridge until the ganache is thick enough to spread.
To make the strawberry ganache, gently heat the double cream in a pan over a low heat until hot but not boiling. Add the chocolate, take the pan off the heat and leave the mixture for 5 minutes to melt the chocolate. Add the strawberry extract and red colouring and stir to a deep pink. Transfer the ganache to a bowl and place in the fridge until the ganache is thick enough to spread.
To assemble the cake, use a serrated knife to level off the sponges. Spread 1 sponge with 3 tablespoons of the strawberry jam, then a little chocolate ganache. Place the second sponge top downwards on top of the first. Using a palette knife, spread the remaining chocolate ganache evenly over the sides of the cake. Spread the top of the cake with the strawberry ganache.
To make the chocolate collar, melt the chocolate in a bowl suspended over a pan of simmering water. When the chocolate reaches 42°C on a sugar or chocolate thermometer, turn off the heat and allow the chocolate to cool to 29°C.
Pour most of the tempered chocolate gradually onto the 2 acetate strips, and use a spatula to smooth it out to an even layer to cover the whole acetate. Pull the coated acetate towards you, to neaten the top edge. You need a small amount (about 25g) of chocolate left over – reserve this to dip the strawberries in. Leave the collar to set in a cool place (not the fridge) for about 30 minutes, until set but pliable. (On exceptionally hot days, you may need to pop the cake in the fridge, but for only 5 minutes, to help the collar set.) Wrap the acetate, chocolate side inwards, around the cake, pressing gently, then refrigerate for 15 minutes, or until ready to serve.
Dip the strawberries into the leftover tempered chocolate (re-melt if necessary) and place them on top of the cake. Drizzle any remaining chocolate over the cake, if you wish. Carefully peel away the acetate to reveal the chocolate collar.