Sift flour and custard powder into a mixing bowl. Stir in baking powder, bicarbonate of soda and salt. In a mixer bowl, cream margarine, sugar and corn oil until light and creamy. Beat in eggs, one at a time. Stir in honey and essence. Add in sifted flour mixture to blend into a smooth batter.
Line an 18-20cm diameter deep bamboo basket with greased greaseproof paper. Pour batter into prepared basket and sprinkle over some melon seeds. Steam over rapidly boiling water for 40-45 minutes.