175g butter, softened, plus extra for the tin
250g plain flour
75g caster sugar
For the caramel
100g butter or margarine
100g light muscovado sugar
2 x 397g cans condensed milk
For the topping
200g plain or milk chocolate, broken into pieces
Heat oven to 180C/160C fan/gas 4. Lightly butter a 33 x 23cm Swiss roll tin with a lip of at least 3cm. To make the shortbread, mix the flour and caster sugar in a bowl. Rub in the 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 mins or until firm to the touch and lightly browned. Leave to cool in the tin.
To make the caramel, tip the butter, sugar and condensed milk into 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 mins or until the mixture has thickened. Pour over the shortbread and leave to cool.
For the topping, melt the chocolate in a bowl over a pan of hot water. Pour over the cold caramel and leave to set. Cut into squares or bars to serve.