Mix together the sugar, lime zest and vanilla extract.
Cream the butter into the sugar mixture until soft.
Stir in the flour and bring the dough together with your hands.
Add the white chocolate chips and mix them in.
Bring the dough back together.
Grease a 7 inch tin and preheat the oven to 180 degrees.
Press the dough into the tin evenly
Score lines where you want to separate the slices, and use the back of a fork to crimp the edges.
Bake for 15-20 minutes until just golden brown. It will harden as it cools, don't worry if it still feels soft.