Position the oven shelves towards the top of the oven. Preheat the oven to 200°C/fan oven 180°C/Gas Mark 6. Lightly grease two baking trays with a little Trex.
Beat the egg, vanilla extract and sugar together using a wooden spoon.
Put the Trex into a mixing bowl with 150g (6oz) flour and the salt. Beat together with a wooden spoon until smooth and creamy. Gradually beat in the egg and sugar mixture. Add the remaining flour and polenta or semolina and mix together to make a smooth dough.
Turn out onto a lightly floured work top and roll out with a floured rolling pin until 5mm (¼ in) thick. Use 2 or 3 different-sized heart-shaped cutters to stamp into biscuits. Transfer to the prepared baking trays.
Bake for 16-18 minutes until light golden brown. Cool for a few minutes on the trays, then transfer to a wire rack to cool completely.
Mix 75g (3oz) icing sugar with a little water to make a stiff icing. Pipe carefully around the edge of the biscuits using a small icing bag fitted with a fine plain icing tube. Leave to set. Mix the remaining icing sugar with a little water and 1-2 drops of pink colouring to give a pale pink icing. Spread over the biscuits one at a time, so that it reaches the piped edges. Leave until set.
