Heat the oven to 180c/160c/fan/gas 4. Lightly grease your cake tin with an tip at least 3cm
To make the shortbread, mix 250g of the plain flour and the 75g caster sugar in an bowl. Rub in the 175g of the softened butter until the the mixture resembles like breadcrums
Knead the mixture together until it forms a dough, then press into the base of the prepared tin.
Pick the shortbread lightly with an fork and bake for 20 mintutes or until it is frim to touch and is very lighted browned,leave it to cool in the tin.
To make the caramel, place 100g butter or margarine, next add the 100g light muscoado sugar and the 397g of the condensed milk in a bowl and heat gently until the sugar has dissolved.
Bring to the boil, keep stirring all the time then reduce the heat and simmer very gently stirring cotiously for about 5- 10 minutes or until the mixture has thicked slighly, pour over the shortbread and leave to cool
For the topping melt 200g plain or milk chocolate slowly in a bowl over hot water in a bowl over the cold carmel and now you can eat it.