Bring enough water in a steamer to a boil. Place a greased 20cm square or round cake pan in the steamer to warm up. This enables the kuih to be cooked evenly.
Mix the combined flours with 200ml general santan and leave to soak for 10 to 15 minutes. Add the rest of the general santan into the sugar and the yam. Blend well together. Strain the soaked flour mixture into the yam mixture. Add colouring and alkaline water.
Cook mixture in a large non-stick saucepan over low heat, stirring constantly with a wooden spatula until the batter begins to thicken. Pour batter into prepared cake pan. Level surface with a plastic spatula and steam over rapid boiling water for 25 minutes.
Meanwhile, prepare the white top layer. Combine rice flour, tapioca flour, sugar and salt into a mixing bowl. Stir in santan and water. Pour over cooked yam layer. Steam again for a further 15 to 20 minutes. Cool the kuih thoroughly before slicing into diamond-shaped pieces.