Chocolate Biscuit Cake

12 oz good quality chocolate (not too milky, not too bitter: 50% is good)
4 oz butter
1 can (397g) Condensed Milk
10 oz biscuits: Rich Tea, or Digestive, or a mixture of both, broken into pieces but not into dust

Melt chocolate and butter slowly together in a large bowl, over, (not in,) barely simmering water.

When runny all through, mix in the condensed milk.

Then the broken biscuits

Spread into shallowish tin lined with silicone paper.  (not flat like a Swiss-roll tin, but not deep as a cake tin. You know the sort of thing: many small roasting tins are about the right size and depth)
Cool.