Grease a shallow 9" tin and preheat oven to 180°C.
In a food processor, combine the butter, castor sugar and flour and process the mixture by pulsing the machine until the mixture reassembles fine breadcrumbs.
Add the egg yolk and milk. Process until the mixture forms a dough.
Press the dough into the base of the tin
Bake in the oven for 25-30 minutes. Allow it to cool in the tin.
To make the caramel filling, place all the caramel ingredients into a saucepan and melt over a low heat.
Once the butter is melted and the sugar is dissolved, turn up the heat and slowly bring it to the boil.
Stir the caramel as it boils for 5 minutes.
Take it off the heat and allow it to cool for 10 minutes.
Pour the caramel over the base and spread it evenly.
Allow this to set while you make the chocolate topping.
Melt the chocolate and butter in a saucepan (or you can use a microwave).
Pour the melted chocolate over the caramel and spread until you have a nice even layer.