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For the sponge:
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1. Preheat the oven to 180 C/160 C fan. Line a Swiss roll tin (23cm x 33cm) with parchment paper.
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2. Melt the chocolate in a heat safe bowl over a pan filled with a little water which should be just about at simmering point. Allow the chocolate to melt slowly and make sure that the water doesn't touch the chocolate in the bowl. Once it has completely melted, carefully remove it from the heat and allow it to cool. Chocolate is quite sensitive and the first time I did this, I didn't allow it to cool for long enough and the chocolate seized up. What I find quite useful is to transfer the melted chocolate from the bowl which has been suspended over the water to another bowl. This may add to the washing up but it helps cool the chocolate and prevent you wasting a whole bar of chocolate. Make sure to use a spatula if transferring it to another bowl to make sure you don't leave any chocolate behind.
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3. While the chocolate is cooling, whisk the eggs, vanilla extract and sugar in a large bowl using a mixer or whisk, until thick, frothy and pale yellow in colour. You will know that it is ready when the whisk leaves a trail when run through the mixture. This will take about 10 minutes by hand and about 5 minutes if using an electric mixer. Carefully fold in the cooled white chocolate and then the flour. Remember that you want the sponge to be light so be gentle and try to keep in as much air as possible.
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4. Pour the mixture into the lined tray, smooth the top so that the batter covers the entire pan, and bake in the oven for 15-20 minutes until it feels spongy. It won't look like a traditional sponge cake so don't be alarmed by this and don't be tempted to continue cooking it as this will likely lead to a cracked Swiss roll. While the sponge is baking, place a large sheet of parchment paper on a work top and sprinkle with caster sugar.
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5. Once ready, remove the sponge from the oven and quickly flip the tin over onto the sugared paper. Remove the tin and then gently peel off the paper from the bottom of the sponge. From the short end, loosely roll up the sponge and sugared paper together to form the traditional roll. Leave it rolled up to cool completely.
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For the filling and topping:
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1. Melt the white chocolate over a pan of hot water as before. Again allow the melted chocolate to cool a little. Lightly whip the double cream. You don't want it to be too thick but you also don't want it to pour out of the sides. Then gently fold the white chocolate (holding back about 1-2 tablespoons for drizzling on top) and the seeds from the vanilla pod or vanilla bean paste (if using) into the cream. Next fold in half of the pistachio pieces.
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2. Unroll the Swiss roll and spread the cream over the sponge leaving about a 1cm gap around the edges. Next take the raspberries (holding back 5-6 full raspberries to put on top of the Swiss roll) and lightly break them in your hands over the cream. You don't want a pulp but just some bits of raspberries and some of the juice scattered over the cream. Finally, sprinkle over half of the remaining pistachio pieces and then carefully re-roll the Swiss roll.
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3. To decorate, using a spoon or fork, drizzle over the remaining melted chocolate. Scatter over the remaining pistachio pieces and top with the reserved whole raspberries.
