Preheat the oven to 170°c
Beat the butter and icing sugar together until pale and fluffy. Now add the flours and vanilla extract and mix until it forms a dough.
Take a large piece of clingfilm and lay it on the work surface. Place your dough on top and roll it into a sausage shape about 5cm in diameter.
Now roll the clingfilm around it and place in the fridge to firm up for 15 minutes.
When chilled, slice the dough 1cm thick and place cut side down on a lined baking sheet.
Place in the oven and cook for 12-15 minutes until golden. Gently remove them and cool on a wire rack.
Melt the chocolate and then spread it over one half of the biscuit, place on some greaseproof paper to set and then decorate to your heart’s content! If you are sprinkling over decoration, better to do it when the chocolate is still wet.