Preheat the oven to moderate 180c ((350f/ Gas 4).
Lghtly grease a 20x30cm baking tin and line with baking paper, hanging over the two long sides.
Place the dates in a saucepan with 200 ml of water.
Bring to a boil, then reduce the heat and simmer gently for 10 mins.
:- Make sure the water doesn`t evaporate completely.
Add the bicarbonate of soda and leave to cool.
Place 185 g of the butter, the flour, vanilla, baking powder, eggs and 75 ml of the milk in a food processor and mix in short bursts for 1 min, or until well blended.
Add the dates and pulse to blend.
:- DO not overprocess!
Place the mixture in the tin and bake for 20 mins, or until a skewer inserted in the centre comes out clean.
Set aside to cool.
Place the remaining butter and milk and the brown sugar in a pan and heat gently to dissolve the sugar.
Add the icing sugar and mix well.
Spread over the cooled slice and sprinkle with the walnuts.