Put the flours in a bowl or food processor. Add the butter and rub in with the fingertips or process until the mixture resembles coarse breadcrumbs. Add the sugar and mix or blend to a dough. Wrap in clingfilm and chill for 1 hour.
Roll out the dough on a lightly floured surface to 5 mm (¼ inch) thick. Cut out about 24 rounds using a 5 cm (2 inch) plain biscuit cutter, re-rolling the trimmings to make more. Place slightly apart on a large greased baking sheet and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 15 minutes until very pale golden. Transfer to a wire rack to cool.
Make the filling. Beat together the icing sugar, butter and vanilla in a bowl until smooth. Add the measurement water and beat until pale and creamy. Sandwich the biscuits together in pairs using the buttercream and jam. Serve dusted with sifted icing sugar.