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1
Sprinkle gelatin powder over 3/4 cup of cold water (or according to instructions on packet) or place sheets in water. Leave to soften while you make the raspberry mixture.
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2
Put the raspberries and icing sugar in a small saucepan over a medium heat and stir until just boiling. Turn the heat down and let the berry mixture simmer for several minutes - if you have a thermometer, remove from the heat when the mixture reaches roughly 75C (160-170F). Then remove from the heat and let cool for 5-10 minutes. You can add more icing sugar if you prefer a sweeter mousse, but I think a slightly tarter mousse works best here.
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3
Pour the double cream into the bowl of a stand mixer fitted with the whisk attachment (or a large bowl, if whisking with an electric whisk) and whip until soft peaks form.
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4
If using gelatine sheets, remove them from the water and squeeze the excess water out of them. Place gelatine into the saucepan with the raspberry mixture, and whisk until the gelatine is dissolved and combined.
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5
Add 1 cup (or about 10 tablespoons) of the cream into the berry mix and whisk until combined. Fold this mixture back into the rest of the whipped cream until well combined.
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6
Put the raspberry mousse in the refrigerator (if you used a stand mixer, put it into a different bowl because you will need the mixer bowl later!) until needed later on.
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7
Place the eggs and sugar inside the bowl of a stand mixer fitted with the whisk attachment (or a medium-sized bowl, if using an electric whisk) and whip until pale and fluffy.
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8
Fold in the flour, making sure to fully incorporate it (i often quickly mix it in with a teaspoon - you don't lose too much air because of the size of the spoon, but it helps if there are pockets of unmixed flour.)
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9
Fold in the cardamom and pistachio, again making sure to fully incorporate it.
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10
Line a 9x13 inch (23x33 cm) swiss roll tin with greaseproof or parchment baking paper. Pour the mixture into the tin, making sure to spread it evenly into all four corners.
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11
Put the cake in the oven, and bake for about 7-10 minutes or until golden brown on top, and springy to the touch. While the cake is baking, put a separate sheet of greaseproof or parchment paper on a cooling rack (the same size as the swiss roll tin).
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12
When the cake is out of the oven, immediately turn it out onto the cooling rack. Score a line about 2 centimetres from one end (this makes it easier to start the roll) and roll up the cake while still hot. Keep it there for a few seconds, then unroll and leave to cool. This helps to roll the cake back up when cool.
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13
While waiting for the cake to cool, make your rosewater drizzle. I didn't use exact measurements here apart from the amount of rosewater, so form a paste in a small bowl with icing sugar and water which is just thicker than a drizzle, then add the rosewater. It should be at a good consistency to drizzle over the cake - but you can add more or less sugar depending on preference. You can also add a little red or pink food colouring if you want it to take on a rosy hue.
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14
When the cake is cool, spread the raspberry mousse over it and make sure it is even - mine was just under a centimetre thick. You may have a little left over - don't spread it on too thickly, or it will be difficult to roll.
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15
Roll up the cake tightly, then cut off the ends as this allows you to see the roll inside better. (I would recommend eating the ends at this point, and tasting your handiwork.)
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16
Drizzle the rosewater icing over the top, and serve however you like - perhaps with a citrussy drink such as cloudy lemonade.
