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 20cm by 26cm 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.
Brush the sponge cake with the stout syrup until the top layer is covered. Spoon 300g of stout and blackcurrant 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 and brush the sponge with the stout syrup. 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. Brush the sponge with stout syrup and 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 knife into the water to warm it. You will also need a towel or kitchen roll to wipe the knife 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, gold leaf, reserved salted caramel or fruit.