Lightly grease sides of several heatproof cups. Scatter a little grated coconut over the base of each cup.
Steam sweet potatoes till soft and tender. Remove and mash finely. Sift the flour and baking powder together; set aside.
Place egg yolks and egg whites into two separate mixing bowls. Whisk the egg yolks and half the sugar until light and thick. Add sifted flour mixture, evaporated milk and mashed sweet potato. Whisk until well combined and smooth.
Whisk egg white until just frothy. Add cream of tartar and remaining sugar. Whisk until mixture is stiff and holds its shape.
Fold egg yolk mixture into egg white mixture in two to three batches using a metal spoon. When well blended, spoon batter into prepared cups and steam for 10-11 minutes or until cooked through.