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1. Place rhubarb, sugar and water in a small saucepan and simmer over a low heat until the rhubarb is tender. Cool.
2. In a food processor, combine ground hazelnuts, flour, custard powder, second measure of sugar, spices, salt and baking powder.
3. Pulse to combine, add butter and pulse until texture resembles fine breadcrumbs. Add egg and vanilla and pulse again until dough comes together.
4. Tip onto a lightly floured bench and bring together by hand to form a smooth ball of very soft dough. 
5. Cut off a third, wrap in cling film and refrigerate.
6. Press remaining dough into the base and sides of a 20cm-deep loose-bottomed tart tin.
7. Spoon rhubarb into base of tart. Roll out chilled dough into a rectangle and cut into long strips 2cm wide.
8. Lattice the top of the torte with the strips, pinching them into the dough at each side.
9. Bake in a preheated 180 degC oven 35-40 minutes until pastry is golden brown. 
10. Rest in tin for 15 minutes before unmoulding onto a serving platter. 
    Drench with icing sugar and serve warm with cardamom-spiced yoghurt.

Cardamom spiced yoghurt

1. Combine all ingredients in a small bowl.