Heat oven to 180C
Sift the flour and spices into a bowl.
Beat together the butter and sugar until light and fluffy.
Dissolve the bicarbonate of soda in 200ml boiling water, gradually add to the sugar and butter mixture.
Add in the flour slowly, then add the eggs, mixing briefly and in the end, add the buttermilk.
Fold in the 3/4th of the rhubarb.
Pour in a greased and lined tray (I used silicone) and spread the rest of the rhubarb on top.
Bake for 50-60 minutes till a skewer comes out clean.
Dust with icing sugar.