Using a wooden spoon or electric mixer, beat the butter with the icing sugar until pale and fluffy. Add the vanilla and beat for a few seconds. Sift the fl our and cornflour together into the bowl and mix until smooth.
Paint a straight line of red food colouring on the inside of the piping bag, from the tube to the opening of the bag. Spoon the biscuit mixture into the piping bag.
Pipe into 32 swirls on the baking sheet. Each swirl should be about 5cm across and 2.5cm high. Chill in the fridge or freezer for 15 minutes to firm. Meanwhile, preheat the oven to 180°C/350°F/gas 4. Bake the biscuits for about 12 minutes or until they are pale golden. Leave for a few minutes on the baking sheet to set slightly before transferring to a wire rack to cool.
To make the filling, heat the milk with the fl our in a small pan, whisking constantly until the mixture boils and thickens. Whisk over very low heat for a minute to cook out the taste of the flour, then pour the mixture onto a plate. Cover closely with cling-film to prevent a skin from forming and leave until completely cold.
Beat together the sugar and butter until very pale and fluffy – about 10 minutes by hand or 4 minutes with an electric mixer. Add the thickened milk mixture and beat until light, creamy and almost white. Add the vanilla. If the mixture is very soft, cover and chill until it is firm enough to pipe.
To assemble, spread a little jam on the bases of half the biscuits. Pipe small swirls of the filling onto the bases of the remaining biscuits and sandwich pairs together. Lightly dust with icing sugar.
This recipe is brought to you from 'The Great British Bake Off' by Linda Collister published by Random House
