Peel the potato and cut into smaller pieces and steam until they are fork tender and can be easily mashed. Transfer the purple sweet potato, coconut cream, and food coloring in a blender and blend until smooth. It will be thick, but smooth. I get a better result when I do this part first before combining them into the batter. Set aside.
In the same steamer, steam the grated coconut with 1/4 tsp of salt for 10 minutes. Set aside
I used muffin cups to make this putu ayu. Brush with some cooking oil on the base and the side of the cups. Portion out the steamed grated coconut into the muffin cups. Make sure to PRESS the coconut down firmly with your fingers on the base of the cups so when you invert the cake the grated coconut won't come "loose"
Place the eggs and sugar in a mixing bowl. I used a stand mixer. Beat on medium speed for 5 minutes and then increase to high speed and beat for another 10-15 minutes or until they are thickened and the color turns pale. It is important to beat it for that long so your cake will be light and soft. The volume will double up too. Reduce the speed to medium and in purple yam, coconut cream mixture and stir until all mixed in. Add few drops of purple food coloring if using. It depends on how deep you want the color to be. Reduce the speed to low and add vanilla, salt, and all-purpose flour. The idea is just to mix in the flour. You don't want to overbeat the flour. Once they are all mixed well, turn off the mixer and portion out the batter into the muffin cups you have prepared with coconut earlier, about 3/4 cup full. Steam for 15 minutes
Remove from the steamer and wait for 5 minutes before removing from the cups. If necessary, run a knife at the side of the cup to loosen the cake. They shouldn't be very sticky or difficult to remove. Invert into a serving platter. The coconut portion should be on top as shown in the photos. Serve warm or room temperature
You can store the rest in the refrigerator in an air-tight container up to 3 days. Just reheat in the microwave for few seconds (I'm not sure because we don't use microwave) or in the steamer for 5 minutes
You don't have to use a purple food coloring. Without food coloring, the color will naturally be dark purple after steaming