Grease 2 baking sheets. Put the flour and butter in a food processor and blend until the mixture resembles breadcrumbs. Add the sugar, egg yolks and vanilla bean paste and blend to make a smooth dough. Wrap in clingfilm and chill for an hour.
Roll out half the dough on a lightly floured surface to about 2.5 mm (⅛ inch) thick, and cut out heart shapes using a 4 cm (1¾ inch) heart-shaped cutter. Space slightly apart on a baking sheet. Roll out the remaining dough and shape more hearts. Put on the second baking sheet and reroll the trimmings for extra. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for about 8 minutes, or until just beginning to turn golden around the edges. Transfer to a wire rack to cool.
Make the buttercream by beating together the butter and icing sugar in a bowl until smooth. Add the boiling water, the pink food colouring and enough of the rosewater to give a delicate flavour, then beat until pale and creamy. Use the buttercream to sandwich the biscuits together. Serve lightly dusted with icing sugar.