To prepare palm sugar syrup: Combine palm sugar, water and screwpine leaves in a saucepan. Cook gently over low heat for a while until sugar dissolves. Discard screwpine leaves and strain the syrup. Leave to cool completely.
Stir yeast, sugar, flour and water into a small bowl. Cover and stand for one hour to allow it to ferment.
Whisk eggs and soft brown sugar until creamy in texture. Gradually stir in flour followed by fermented yeast mixture.
Add the palm sugar syrup, evaporated milk, melted butter and walnuts to mix, then rest batter for five minutes.
Spoon batter into greased moulds until three-quarters full and steam over rapidly boiling water for six to seven minutes.
Remove steamer cover and scatter some extra chopped walnuts on top of each cake. Cover and steam for a further four to five minutes or until cake is cooked through.