Heat the oven to 180C/160C fan/gas 4. Lightly butter or oil a swiss roll tin or sturdy baking tray – ours was 24 x 34cm. Line the base and sides with baking parchment, ensuring the sides are at least 2cm tall. The easiest way to do this is by snipping a small diagonal cut into each corner of the parchment and folding them in against the tin.

Heat a small pan of water until bubbling, then turn the heat to low and set a heatproof bowl on top, ensuring the bowl doesn’t touch the water. Tip in the chopped dark chocolate and 75ml warm water and melt for 5-10 mins, stirring every now and then, until you have a smooth, even consistency.

Separate the eggs into two large bowls, add a pinch of salt to the egg whites and beat using an electric whisk for 4-5 mins until doubled in size and holding soft peaks.

Add the sugar to the egg yolks and whisk using the electric whisk (no need to wash it first) for a few minutes to create a mousse-like consistency. Slowly pour in the melted chocolate and beat until smooth, about 1 min.

Using a large metal spoon, add a scoop of the beaten egg whites to the chocolatey yolks and mix to loosen the consistency. Add the remaining egg whites, and gently fold the ingredients together, scooping the spoon under the mixture then lifting and cutting through while turning the bowl, until you have a fluffy, chocolate mousse-like consistency, with no pockets of egg white. Be careful not to overmix as you want to retain as much air in the mixture as possible. Pour the mixture into the prepared tin, gently ease it out to the edges using a spatula, and level the top. Bake for 18-20 mins until a skewer inserted into the centre of the cake comes out clean.

Remove from the oven and leave to cool for 5 mins. Meanwhile, lay a large sheet of baking parchment on your work surface and dust with cocoa powder. In one swift movement, flip the cake onto the cocoa-dusted surface. Cover with a tea towel and leave to cool for 30 mins-1 hr.

Meanwhile, for the filling, tip the raspberries into a pan with 1 tbsp of the icing sugar. Bring to a simmer over a medium heat and cook for 2-3 mins until juicy. Remove from the heat and set aside to cool, then chill until you’re ready to assemble.

Pour the cream and remaining 2 tbsp icing sugar into a bowl. Beat using an electric whisk until the cream holds peaks, about 1-2 mins – too soft and it will spill out of the cake, too firm and it will be difficult to spread, so do keep an eye on it.

Remove the tea towel from the cake. With one of the shorter ends facing you, spread the pistachio butter over the surface, leaving a 1-2cm border on the nearest edge to you. Top with the whipped cream, then spoon over the cooled raspberries and their juice. Use the parchment to help you fold this shorter edge back on itself into the cake, then roll the roulade up tightly, keeping the parchment on the outside the whole time and using it to help secure it into a nice tight roll. Chill for at least 30 mins. Will keep frozen for up to a month.

If frozen, defrost the roulade in the fridge for 24 hrs. When you’re ready to serve, unwrap the roulade. To decorate, break off 25g white chocolate and melt in short bursts in the microwave, for about 1 min, or following the same method in step 2. To make chocolate curls, hold a knife at a 45-degree angle to the surface of the chocolate bar and pull it towards you. Drizzle the melted chocolate over the roulade, then pile milk and white chocolate curls on top and scatter over the chopped pistachios.

Serve in slices, with extra cream on the side, if you like. Will keep chilled for up to five days.