1. Grease a 20 x 20cm/8 x 8in deep baking dish.
2. Preheat oven to 170C/325F/gas mark 3.
3. Melt the chocolate and butter in a bowl over a pan of simmering water.
4. Put the eggs and milk in a large mixing bowl and beat lightly.
5. Add the sugar and beat a little more.
6. Stir in the chocolate and butter mixture.
7. Sift the flour, salt and baking powder into the mixture and beat until well blended.
8. Stir in the vanilla extract and finally fold in the chopped nuts.
9. Pour the mixture into the greased baking dish and bake in the pre-heated oven for 30-35 minutes until risen and springy in texture.
10. Turn out on to a cooling rack and, when cold, cut into 5cm/2in squares.