Sift flours and custard powder into a large mixing bowl. Stir in sugar; mix. Add melted butter, essence, treacle and milk. Beat in eggs and use a hand whisk to mix until mixture is smooth.
Sift in baking powder to combine. Pour batter into a lightly greased 20?22cm round cake tin.
Sprinkle some olive nuts over the batter or, if preferred, use melon seeds.
Put into a steamer and steam over rapid boiling water on high heat for 30?35 minutes or until cooked through.
Remove steamed sponge from the steamer and set aside to cool completely before cutting into pieces.