Preheat the oven to 180˚C (160˚C fan oven, 350˚F, gas 4). Grease and line an 18cm (7inch) straight-sided square cake tin. In a bowl, beat the butter and sugar together until soft and creamy. Sift the flour on top and stir into the creamed butter and sugar until the mixture forms a crumbly dough, finally stir through the Chocolate Chunks.
Pile the crumbs into the tin and then press the mixture evenly across the base of the tin using the back on a spoon until completely covered and smooth. Prick all over with a fork and bake for 25-30 minutes until golden and just firm to the touch. Leave to cool in the tin.
Put the butter, condensed milk, syrup and Orange Extract in a saucepan and heat gently until mixed. Raise the heat and bring the mixture to a gentle boil, stirring all the time, and simmer gently for about 5-7 minutes until the mixture has thickened and is a creamy fudge colour. Pour over the shortbread and allow to cool.
Break up the Chocolate and place in a heatproof bowl. Put the bowl over a saucepan of barely simmering water and leave to melt. Remove from the water and cool for 10 minutes.
Spread the melted Chocolate evenly over the top of the orange caramel. Tap the tin on the work surface to smooth and put in a cool place or the fridge for about 10 minutes until the Chocolate begins to set.
Whilst the topping is still slightly soft, score the topping right through with a sharp knife into 12 equal squares, then chill for a further 30 minutes.
To serve, carefully lift out the contents of the tin using the paper tin lining. Place on a board and using a large bladed knife, cut into 12 pieces. Your chocolate orange millionaire shortbread is now ready to serve and enjoy!