Ottolenghi’s rhubarb and custard yo-yos (sandwich biscuits)

Method

1. Heat the oven to 180°C/160°C fan/gas 4 and line a small baking tray with baking paper. To make the rhubarb icing, spread the rhubarb out on the lined baking tray and roast for 30 minutes or until softened. Remove from the oven and allow to cool before transferring to the small bowl of a food processor. Whizz to a purée, then add the butter. Sift in the icing sugar, add the lemon juice and whizz for a couple of minutes: it seems like a long time, but you want it to thicken.
2. Transfer to a small bowl and chill in the fridge for 2 hours to firm up. You don’t want the icing to be at all runny, so add a little more icing sugar if necessary; it needs to hold when sandwiched between the biscuits.
3. To make the dough, sift the flour, custard powder, icing sugar and salt into the bowl of an electric mixer fited with the paddle attachment. Mix on a low speed to combine. Add the butter and mix on a low speed until the mixture resembles breadcrumbs. Add the vanilla extract, increase the speed to medium and beat for about 30 seconds until the dough comes together.
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4. Reduce the oven temperature to 170°C/150°C fan/gas 3. Line 2 baking trays with baking paper. Pinch off small bits of dough and use your hands to roll them into 3cm balls: you should have enough for 30, weighing about 15g each. Put them on the lined baking trays, spaced about 4cm apart. Dip the back of the prongs of a small fork in flour before gently but firmly pressing down onto each ball: they will expand to about 3.5cm wide, but don’t press all the way to the bottom; you just want to create firm lines in the dough rather than force them to spread out.
5. Bake for 25 minutes, rotating the trays halfway through, until the biscuits are dry on the bottom but haven’t taken on too much colour. They’ll be relatively fragile when warm but still firm to the touch. Set aside on the trays to cool for 5 minutes before cooling completely on a wire rack.
6. To assemble, sandwich pairs of biscuits together with the icing, with the forked sides facing outwards. You should use about 15g icing in each biscuit. It will seem like a lot, but trust us – the biscuits can take it.