Place the jaffas in a bag (thick plastic is best, line a zip lock bag). Use a meat mallet to break them into halves and quarters, either is fine.
In a microwave safe bowl, combine the chocolate and butter. Melt in the microwave (stir every 20 seconds) until just smooth. Cool for 5 minutes.
Stir in the sugar and flour. Fold in the jaffas.
Spread into prepared tin, bake at 180oC for 30-35 minutes, the top should be just firm to touch. Let stand for 15 minutes (it will continue to cook while in the pan, so don't over cook it or they will be dry).
Once cold (this will take about 2 hours) remove from the pan and cut into 12.