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Tip the rhubarb, sugar, orange zest and juice and vanilla bean paste into a saucepan over a medium heat and cook, stirring often, until the rhubarb has completely broken down and you can draw a line through the mixture with a spatula, about 10-15 mins. You can remove a few pieces of rhubarb after 2 mins to decorate, if you like. Once thick, glossy and pink, transfer to a heatproof bowl, cool completely and keep chilled until needed. Will keep chilled for up to three days.
For the shortbread, beat the butter, sugar, vanilla bean paste, cardamom and a large pinch of sea salt together using an electric whisk until light and fluffy. Add the egg and briefly whisk again to combine. Sift over the flour and add the pistachios. Stir until you have a shaggy dough, then bring it together with your hands, being careful not to overwork it.
Tip the dough out onto a large sheet of baking parchment and use the parchment to help you roll it into a tight log roughly 25cm in length. Chill in the fridge for 1 hr or freezer for 30 mins. Will keep wrapped and frozen for a month.
Heat the oven to 180C/160C fan/gas 4. Slice the chilled dough into 1cm-thick rounds and arrange on a baking tray lined with parchment. Bake until lightly golden and set, about 12-15 mins. Leave to cool on the tray.
For the fool, whisk the cream and sugar together to soft peaks, then fold in the yogurt and most of the rhubarb compote. Divide the rest of the compote between four glasses or bowls, then top with the fool and any remaining compote. Serve with the shortbread for dunking.
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