Preheat oven to 160°C (140°C fan forced) and lightly grease a lamington pan (23cm x 33cm) and line with baking paper.
Place pastilles in a mixing bowl over a saucepan of simmering water, stirring occasionally until melted. Remove mixing bowl from heat and allow chocolate to cool slightly.
Meanwhile, using an electric mixer, beat butter and sugar together for 5 minutes or until light and fluffy. Add eggs one at a time, beating well between each addition. Add the flour to the egg mixture and using a large metal spoon gently fold into mixture. Add cooled chocolate and again fold to combine.
Spoon mixture into prepared tin and bake in preheated oven 30 minutes, or until a skewer comes out of the centre clean. Remove from oven and allow to cool in tin for 5 minutes, before turning onto a wire rack to cool.