To make the caramel topping, put the unopened tin in a heavy-based saucepan and completely cover with water. Cover the saucepan with its lid and boil for about 1½ hours, topping up the water level if needed.

Preheat the oven to 180ºC/350F/Gas mark 4, 10 minutes before baking.

Line a 22cm/8inch square cake tin with parchment/greaseproof paper

Cream together the butter and sugar until light

Sift in the cornflour and plain flour and mix to form a smooth dough

Using the back of a dessert spoon, press the mixture evenly into the lined cake tin and prick all over with a fork

Bake for 20 minutes or until just turning golden brown

Put the nuts on to a baking tray and toast them in the oven for 10-15 minutes

Remove the shortbread from the oven and set aside on a wire rack

Remove the nuts from the oven and wrap them in a clean tea towel. Rub the nuts together to remove most of the skins (especially if you're using hazelnuts or 'red skinned' peanuts)

Reserve 9 of the nuts, roughly chop the remainder and sprinkle them evenly across the shortbread

Open the tin of boiled condensed milk (if the contents are quite rigid you can soften it by warming slightly in a saucepan on the stove or decant into a microwaveable container and heat for 20-30 seconds). Pour the caramel over the nuts and spread evenly. Refrigerate while you prepare the chocolate

Break up the chocolate into pieces and put them into a heat-proof bowl

Using a saucepan small enough not to allow the bowl to touch the bottom, fill the bowl with just enough water so that it doesn't come into contact with the base of the bowl

Simmer the saucepan of water until the chocolate has just melted

Pour the chocolate evenly over the top of the caramel

Place the whole nuts on top of the chocolate, one for each portion

Allow to set before slicing into squares & serving