Prepare Ingredients: Separate egg yolks and whites. Mash purple sweet potatoes. Cut some purple sweet potatoes into tiny pieces and coat with cake flour.
Make Egg Yolk Mixture: In a bowl, whisk egg yolks with sugar until pale. Add mashed purple sweet potatoes, coconut milk, cooking oil, and ube extract or purple food coloring. Mix well.
Add Dry Ingredients: Sift cake flour and salt into the egg yolk mixture. Fold gently until combined.
Prepare Meringue: In a clean bowl, beat egg whites with cream of tartar (or lemon juice/vinegar) until foamy. Gradually add sugar and beat until stiff peaks form.
Combine Batter and Meringue: Gently fold the meringue into the egg yolk batter in batches until well incorporated.
Add Purple Sweet Potato Pieces: Fold in the tiny pieces of purple sweet potatoes coated with cake flour.
Bake: Pour batter into a 7-inch chiffon tube pan. Bake at 160°C (320°F) for about 55-60 minutes or until a skewer inserted comes out clean.
Cool and Serve: Invert the pan and let the cake cool completely before removing from the pan. Slice and serve.