Chocolate-filled sponge roll

Method

1. For the pastry cream, melt the dark chocolate either in a microwave or over a bain marie (double broiler).
2. Use an electric mixer to whisk the yolks and sugar together until pale. Stir in the cornflour. Put the mixture in a saucepan over low heat and slowly add the milk, little by little. (If your lowest burner is still quite aggressive, do this over a bain marie or double broiler so the eggs don’t curdle from being heated too rapidly.) Stir continuously with a wooden spoon or silicone spatula until the mixture becomes smooth and thick and coats the back of a spoon, about 10 minutes. Remove from the heat and stir through the melted chocolate. When it is smooth and well combined, cool quickly by spreading the pastry cream out into a shallow, flat container such as a glass lasagne dish or baking tray. Place the plastic wrap right over the top of the pastry cream, so the entire surface is in contact with the plastic wrap. This will ensure the pastry cream doesn’t develop a skin. Keep in the refrigerator until needed.
3. Preheat the oven to 160ºC and line a 23 x 33 cm baking tray with baking paper.
4. To make the sponge, sift the cornflour and flour together. Put the separated eggs in two clean metal or glass mixing bowls, yolks in one and whites in the other. Whisk the egg yolks and the sugar with an electric mixer or electric egg beaters for up to 10 minutes, or until the yolks become very pale and creamy. Clean the beaters very well, then whisk the whites until stiff peaks form. To the creamy egg yolks, gently fold in half of the whites and then half of the flours, and repeat with the remaining whites and flours until they are all combined.
5. Pour the batter into the lined baking tray – the batter should be about 1 cm high. (You can also use a flat tray and spread the batter out to the size you like with a palette knife.) Bake in the oven for about 10–12 minutes, or until the top is very pale golden and springy in the middle.
6. Remove the sponge from the oven and let it cool ever so slightly so you can handle it easily – you still want to work with it while it’s warm. Spread out a sheet of plastic wrap and scatter it evenly with the caster sugar (this helps to stop the sponge from sticking to the plastic). Gently turn the sponge upside down onto the plastic wrap. Remove the baking paper to reveal a spongy soft cake. With a bread knife, trim the edges – this will stop the sides from cracking as they roll. Then, with a pastry brush, stain this side of the sponge evenly with Alchermes (see note for non-alcoholic version). You may have to do a couple of ‘coats’ for a bright pink. Take the cooled pastry cream out of the refrigerator and generously spread it over the top of the pink sponge so that it is about 1.5 cm thick, smoothing out evenly and leaving a 1 cm border around the edges.
7. The rolling part is rather like rolling sushi, if you’ve done it. The tricky part is really in starting the roll – the rest is all about the right amount of pressure (not too tight, but not too loose). Picking up the short end of the sponge with the help of the plastic wrap, carefully roll the entire thing up firmly and then secure by wrapping completely in the plastic wrap. Keep in the fridge to chill for 1 hour or overnight.
8. Remove the plastic wrap carefully, dust the top with icing sugar and serve in thick slices.