First make the ganache, put the chocolate in a medium sized bowl, cut the butter into cubes and put it in a separate bowl.
Place the whipping cream and blackberry puree into a saucepan and heat until it comes to a boil. Remove the pan from the heat.
Pour the hot liquid over the chocolate and stir gently until all of the chocolate has melted, then add the butter and stir gently until all melted.
Leave to one side to cool, if not using straight away place it in a bowl or container with cling-film pressed into the surface of the ganache so that no condensation forms. This can be made 2 days in advance.
Next make the ganache glaze, put the cream, sugar and liquid glucose in a saucepan and bring it to the boil.
Add the hot cream mixture to the chopped chocolate, mixing until the chocolate melts and the ingredients come together.
Pour into an airtight container and cover with cling-film, pressing to the surface of the glaze so that condensation or a skin doesn't form.
When cool place the glaze in the fridge and use within 4 days.
Next make the creme au beurre, put the caster sugar and water in a saucepan, bring to the boil and heat to 121C.
Put the egg yolks in the bowl of an electric mixer and whisk, slowly pour in the hot syrup and whisk until thick and cool.
Gradually add the softened butter while still whisking. When all of the butter has been incorporated, continue to beat until light and aerated.
Add the blackberry puree gradually and taste as you go along, until you get the strength of flavour desired without compromising the texture.
To make the biscuit joconde preheat the oven to 200C/180C Fan and line 3 baking trays that are at least 24cm x 30cm (it doesn't matter if they're a little bit bigger as you will trim the finished cake).
Put the sifted icing sugar, ground almonds and sifted flour in a large mixing bowl, add the eggs and beat until the mixture is light and pale.
Melt the butter gently, do not let it boil or simmer, add the melted butter to the egg mixture and beat until it is fully mixed in.
Put the egg whites and caster sugar in the bowl of a stand mixer and whisk until at firm peaks stage.
Fold the meringue into the egg mixture until fully mixed, weigh the cake mixture (the easiest way to do this is to tip the mixture back into the bowl of the mixer and weigh it) and then split it between the 3 baking sheets.
Bake in the preheated oven for 12-15 minutes until golden brown and the sponge springs back when pressed gently.
Before you start, make sure all of your ingredients are cool or at room temperature, give the creme au beurre a whisk to make sure it's velvety and smooth or use it straight after you've made it.
Cut your three sponge cakes down to 22cm by 28cm, or the largest size you can by keeping them all the same size and place one of the sponges on the greaseproof paper you cooked it on and then place this on top of a chopping board or any surface that the whole cake can fit on and be transported into the fridge. Also make sure you have space in the fridge for the cake to fit.
Spread 300g of blackberry ganache on top of the sponge cake and spread it out evenly. Put the cake in the fridge and rest it for 20 minutes.
Weigh 300g of creme au beurre out and spread it evenly over the ganache. Place a second sponge on top. Spoon the rest of the ganache on top of the sponge and spread it out evenly. Return the cake to the fridge for 20 minutes.
Spread the remaining creme au beurre on top of the ganache and place the final sponge cake on top. Leave to set in the fridge for an hour and a half.
Warm the glaze gently in the microwave or on the hob until it reaches 30C, take the Opera from the fridge and pour half to two thirds of the glaze over the top (you probably wont need all of it) until it is covered evenly. Place the Opera back in the fridge for 10 minutes to set.
Prepare a jug full of hot water and dip a long pastry knife into the water to warm it. You will also need a towel or kitchen roll to wipe the knife clean after each cut.
Take the Opera from the fridge and dry off the warmed knife with the towel. Trim the four edges of the Opera to neaten it up. Making sure to clean the knife in the water and wipe it dry with the towel after each cut.
Be sure to slice downwards and pull the trimmed edges away from the cake, do not saw with the knife, or pull it up or through the cake, this will mess up your lovely layers.
Then cleaning the knife between cuts, divide your cake into portions, I managed to get 12 portions. place these on a tray/plate and decorate however you wish with tempered chocolate decorations and blackberries.