Add carb soda to boiling water, pour over the roughly chopped butter in a large bowl and stir until the butter is melted. Add egg and whisk lightly with a fork to break up the egg and combine with butter and water. Combine flour and sugar and add to the mixture, a couple of spoonfuls at a time, whisking in gradually with a fork until the mixture is smooth. Add dates and walnuts and mix to distribute evenly through mixture.

Grease and lightly flour a medium square cake tin (use one at least 2 inches deep) or a loaf tin, pour mixture into tin and bake at 325-350° for about 45 minutes, or until cooked (check with skewer after about 30 minutes - don't overcook).