Pour the can of sweetened condensed milk into a heatproof bowl and place bowl over a saucepan of simmering water. Cover and cook, over low heat, stirring occasionally, for 60 to 90 minutes or until the milk has thickened and has turned a caramel color. Remove from heat and beat until smooth. Pour the caramel over the baked shortbread and leave to set.
06(Microwave method: Place sweetened condensed milk in a large microwaveable bowl and cook on medium power for about 4 minutes, stirring halfway through baking time. Reduce the power to medium-low and continue to cook for another 8 - 12 minutes, or until the milk has thickened and turned a light caramel color. Remove from microwave and beat until smooth).
07TOPPING:.
08Melt chocolate and butter either in the microwave oven or in a heatproof bowl set over a saucepan of simmering water. Pour the melted chocolate evenly over the caramel and leave to set. Cut the shortbread in to pieces using a sharp knife.
09Store cut bars in refrigerator to keep chocolate firm. Chilled chocolate doesn't cut clean.