Heat the oven to 180C/160C fan/gas 4. Lightly grease an 8inch square tin with a lip of at least 3cm. To make the shortbread, mix 250g plain flour and 75g caster sugar in a bowl. Rub in 175g softened butter until the mixture resembles fine breadcrumbs
Knead the mixture together until it forms a dough, then press into the base of the prepared tin. Prick the shortbread lightly with a fork and bake for 20 minutes or until firm to the touch and very lightly browned. Leave to cool in the tin.
To make the caramel, place 100g butter or margarine, 100g light muscovado sugar and two 397g cans condensed milk in a pan and heat gently until the sugar has dissolved.
Bring to the boil, stirring all the time, then reduce the heat and simmer very gently, stirring continuously, for about 5-10 minutes or until the mixture has thickened slightly. Pour over the shortbread and leave to cool.
For the topping, melt 200g plain or milk chocolate slowly in a bowl over a pan of hot water. Pour over the cold caramel and leave to set. Cut into squares or bars.