Spray a 1 litre (1¾ pint) baking dish lightly with spray oil. Scatter the raspberries over the base of the dish.
Sift the flour and cocoa powder into a bowl and stir in the caster sugar. Make a well in the centre and whisk in the milk, melted butter and eggs to form a smooth batter (it should be quite runny). Pour the mixture into the dish, covering the raspberries.
Make the topping. Combine the sugars and cocoa powder and sprinkle over the top of the chocolate mixture. Very carefully pour the measurement boiling water over the top as evenly as possible.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 40–45 minutes until the pudding is firm to the touch and some 'bubbles' of sauce appear on the top. Rest for 5 minutes, then dust with icing sugar and serve.