Line a 23 x 13 cm / 9 x 5 inch loaf tin with cling film, leaving an overhang on the long sides. Break the biscuits by hand into 2 cm pieces, about the size of a large coin, and put them in a 28 cm / 11-inch bowl with the chopped walnuts.

Combine the milk, sugar, cocoa, butter and salt in a 1.5-quart / 1.4 L saucepan over medium-low heat. Stir for 3 to 4 minutes until the butter melts, the sugar dissolves and the sauce is smooth and glossy. It should steam but not boil. Take it off the heat and stir in the vanilla.

Let the sauce cool for 10 minutes until it feels warm, not hot, on the back of a spoon. Hot sauce softens the biscuits and the cake slumps when sliced.

Pour the sauce over the biscuits and walnuts. Fold with a rubber spatula for 1 to 2 minutes until every piece is coated and there are no dry pockets.

Spoon the mixture into the tin and press it firmly into the corners with the back of a spoon. Level the top, fold the cling film over, and chill for at least 4 hours until completely firm.

Lift the cake out by the cling film and unwrap it. Slice into 12 pieces about 2 cm thick with a sharp knife, wiping the blade clean between cuts.