Cream the butter, sugar and vanilla together in a large bowl until pale and fluffy.
Sift over the flour and cornflour and mix it all together until it is smooth, but hasn't yet formed a ball of dough.
Stir in the crystallised ginger until it is evenly distributed, then bring the dough together into a ball.
Shape the dough into a log about 5 cm (2 in) thick - use your hands and roll it on a lightly floured surface to get it as even as possible.
Wrap the log in a piece of baking parchment, twisting the ends tightly to form a cracker shape. Refrigerate for 1-2 hours until firm.
Meanwhile, preheat the oven to 180°C/160°C fan/350°F/gas mark 4 and line a baking sheet with baking parchment.
Use a very sharp knife to slice the log into approx 8mm thick slices. Place them spaced apart on the baking sheet.
Bake for 20-25 minutes until lightly browned around the edges and on the underside.
Allow to cool on the tray for 5 minutes then carefully transfer them to a wire rack to cool completely.
Melt the chocolate, either in short bursts in the microwave or in a heatproof bowl over a pan of gently simmering water.
Drizzle the chocolate over the cooled biscuits and allow to set before storing in an airtight container.