This is a good recipe to practise your piping skills: buttery biscuits filled with a deliciously sharp rhubarb curd and creamy vanilla butter icing. The ultimate in custard creams!
Heat the oven to 180°C/160°C fan/350°F/Gas 4.
Make the dough. Put the icing sugar, flour and custard powder into the bowl of a food processor and pulse until thoroughly combined. Add the butter and the vanilla bean paste, then blitz to make a smooth dough.
Transfer the dough to a large piping bag fitted with the large star nozzle. Carefully pipe 24 rosettes, each measuring 5cm in diameter, on the lined baking sheets, spacing them well apart.
Chill the biscuits for 10 minutes, then bake for 12–15 minutes, until golden and firm. Leave to cool on the baking sheets for 10 minutes, then transfer to a wire rack to cool completely.
Meanwhile, make the rhubarb curd. Put the rhubarb into a food processor. Blitz to a thick purée, then strain through a fine sieve into a measuring jug – you’ll need 100ml of strained juice.
Break the egg into a mixing bowl and add the sugar and cornflour. Set aside 2 teaspoons of the measured rhubarb juice, and pour the rest into the bowl. Mix thoroughly, then tip into a medium pan. Add the butter and stir continuously over a low heat until the mixture thickens, but does not boil.
Strain the mixture into a clean bowl and, using the tip of a small knife, add a touch of pink colouring. Taste, then stir in the reserved rhubarb juice to taste. Cover, and chill for at least 30 minutes.
Make the vanilla butter icing. Put the butter into a bowl and beat thoroughly with an electric whisk. Gradually beat in the icing sugar, then the vanilla bean paste, and mix thoroughly until light and fluffy, adding a little milk if necessary to create a thick piping consistency. Transfer to the second piping bag, fitted with the medium star nozzle.
Pipe a ring of butter icing on the underside of half the biscuits, just inside the edge. Transfer the rhubarb curd to the disposable piping bag. Snip the end and pipe the curd into the centre of each icing ring. Top with the remaining biscuits (top-sides up). Dust with icing sugar.