Make the shortbread base - Cream together butter and sugar until light and fluffy. Add flour, cornflour, and vanilla extract, mixing until it forms a soft dough.
Press and bake - Press the dough evenly into a lined 8-inch square tin. Bake for 20-25 minutes until pale golden and set. Leave to cool completely.
Prepare the ghosts - Melt white chocolate and pipe blobs onto a baking sheet lined with parchment paper. Using a palette knife or the back of a spoon, press into the blob and drag the knife to smear the chocolate - don't go too thin! Add tiny dark chocolate dots for eyes and mouths, or colour some white chocolate with black food colouring and use this. Let them set in the fridge.
Make the buttercream - Beat softened butter until pale and fluffy. Add icing sugar gradually, then vanilla and black gel food colouring until you get the perfect shade.
Assemble - Spread the black buttercream over the cooled cookie base. Top with your set white chocolate ghosts. Cut into squares and serve.