Separate yolks and egg whites into two mixing bowls, making sure that the egg whites do not contain any trace of the yolks.
Line the base of an ungreased 20cm chiffon cake pan with non-stick parchment paper. Preheat oven to 180°C.
Whisk egg yolks and half of the sugar until light and creamy. Add coconut milk, corn oil and essence. Carefully add sifted flour; fold in lightly and quickly.
Whisk the egg whites until frothy, then add cream of tartar and half of the sugar. (Add sugar gradually, one third at a time.) Beat until the mixture is stiff. Gently fold egg yolk mixture into the beaten egg whites. Carefully mix, lightly and evenly.
Remove one heaped ladleful of batter and stir in dragon fruit puree to mix. Pour 1/3 plain batter into prepared pan. Spoon 2-3 tablespoons of dragon fruit coloured batter over. Pour in another 1/3 plain batter and add dragon fruit coloured batter. Finish off the last portion of plain batter followed by the dragon fruit batter. Use a satay stick to draw around to create a marble effect.
Bake for 35 minutes or until well risen and firm to the touch. Invert the pan immediately and leave to cool completely. To remove the cake from the pan, run a thin blade knife around the sides of the pan. Peel off lining paper.