Mix together the honey and oil. Add the
egg and beat well. Add the bicarbonate of soda to the water and leave
to dissolve. Mix together the flour and salt. Add to the honey mixture
alternately with the bicarbonate of soda mixture. Stir in the rhubarb,
vanilla essence and nuts, if using. Pour into two greased 450 g/1 lb
loaf tins (pans). Mix together the topping ingredients and spread over
the cake mixture. Bake in a preheated oven at 180°C/350°F/gas mark 4
for 1 hour until springy to the touch.