Break the shortbread biscuits into large, uneven lumps (about 1cm / 0.4in).

Roughly chop the nuts into large pieces.

Place the chocolate and the butter, in the glass bowl over the boiling water to create a bain-marie. Stir occasionally.

Once melted, remove from the heat and gradually stir in the sweetened condensed milk. Don’t worry if the mixture separates, keep stirring until the ingredients combine and come together away from the side of the bowl in a dough-like consistency.

Add in the shortbread and nuts and mix together.

Pour into a greased/lined square tin, and when cooled, refrigerate for an hour.

Melt the remaining chocolate and pour over the surface of the cake.

To serve, remove from the tin and cut into serving slices.