Many Indonesian cakes are steamed, like this bolu. When steaming, don’t forget to cover the lid with a clean napkin to prevent vaporized water from dripping onto the dough. This also helps the dough to rise evenly and cook thoroughly.
Cut baking paper into appropriate-sized pieces to fit the individual cup segment. Put into every cup, and place the cup in a steaming pot. Set aside.
Mix egg, sugar, baking soda and baking powder together at high speed. Add vanilla essence; then mix again. Lower the speed, and add the wheat flour alternately with sweet soda water until the batter comes together and rises. Turn off the mixer.
Take ¼ of the batter and mix in food coloring until it is evenly mixed. Set aside.
Fill each of the cup segments almost full with the remaining plain batter. Add brown mixture evenly into each of the partially filled cup segments until the cups are completely filled. Swirl a fork through the top of the batter in each cup.
Steam batter in a tightly covered hot steamer for 20 minutes. Steam until the batter rises and springs into cakes. Lower the heat and let the cakes stand for another 5 minutes in the steamer. Turn off the heat. Open the steamer lid and serve.
Of course, it would be best if you use fresh vanilla sticks instead of the essence
Instead of using food colorant, you can always use fresh pandan or suji for its sweetly smelling aroma as well as a natural green color