Preheat the oven to 180°C, 350°F, Gas Mark 4. Line 2 baking sheets with parchment.
Beat the butter, sugar and condensed milk together until creamy. Add the egg (don’t worry if it looks like its curdling), beat well and then add the flours. Mix to a firm dough. Cover in clingfilm and refridgerate for 30 minutes.
Roll the dough out on a floured surface to the thickness of a pound coin. Cut shapes out.
Transfer the biscuit shapes to the trays and bake for 10 minutes until light golden. Cool completely on a wire rack.