Preheat oven to 180 degrees C. Grease and flour a 23cm square cake tin. In a saucepan, combine water and raisins. Bring to a boil and cook for 5 minutes. Remove from heat and stir in dates and bicarb soda. Set aside.
In a large bowl, cream together 125g butter and 3/4 cup white sugar until light and fluffy. Beat in the eggs one at a time then stir in the vanilla. Beat in the flour, salt and date mixture. Pour batter into prepared tin.
Bake in the preheated oven for 45 minutes, or until a toothpick inserted into the centre of the cake comes out clean. Remove cake from oven and set oven to grill.
In a saucepan, melt 60g butter. Stir in brown sugar, cream and diced walnuts. Spread mixture over warm cake and place under grill until lightly browned, about 3 minutes. Watch carefully - it burns easily.