In a medium-sized pot, heat up the coconut milk until it is just beginning to bubble. Place the lemongrass, kaffir lime leaves, and pandan leaves in muslin bags and tie them closed tightly. Put them in the coconut milk mixture and let simmer for about 10 minutes.
Mix together tapioca flour and rice flour in a large bowl. Add sugar and salt, stirring until all ingredients are combined. Gradually pour in the coconut milk mixture while continuously stirring to make a thick batter.
Pour some of the plain batter into three different bowls – one with cooked beets, one without, and one with vanilla powder.
Layer ⅔ of the plain batter into a greased baking pan. Top with some of the beetroot batter, then add the vanilla powder batter alternately until all batter is used up.
Lightly brush oil over the top of the cake and bake in a preheated oven for 30 minutes. Take out when finished and let cool before serving.