Line a 23cm square pan with wax paper. Arrange a large bamboo steamer or a large vegetable steamer over simmering water. Make sure it is large enough to hold the baking pan.
Separate the eggs. Place the yolks in a large bowl along with the sugar and water. Beat with an electric mixer on medium speed until the mixture has increased about three times in volume. Whisk together the flour and the baking powder; sift this over the egg mixture and fold in gently. Blend in the essence.
Whip the egg whites in a clean bowl to stiff, not dry, peaks. Fold into yolk base. Pour the batter into the prepared pan and smooth out to edges. Rap the pan on the counter to get rid of large air bubbles.
Place the pan into the steamer. Cover the top of the steamer with a kitchen towel and place the lid on top; the towel will absorb any steam that collects on top from dripping onto the cake. Steam for about 20 minutes, or until a toothpick inserted into the centre comes out clean. Cool on a wire rack. Dust with icing sugar.