Ube Chiffon Cake

6 (extra large or 7 regular size) egg whites
3/4 cup sugar

6 (extra large or 7 regular size) egg yolks
1/2 cup oil
1 cup mashed purple yam, (1/4 cup evaporated milk, 1 regular size purple yam)
1/2 cup evaporated milk
1 tsp violet food coloring

dry ingredients
2/3 cup all purpose flour
1/3 cup cornstarch
1 tbsp baking powder
1/2 cup sugar
1/4 tsp salt

1. to make the purple yam, peel off the skin cut into cubes then place in a bowl, put the bowl in casserole with water , not too much water just enough to steam for 30 mins under medium low heat. or you can just put the bowl in microwave for 2 mins just add 2 tbsp water into the yams before microwave to prevent it from being too dry. mashed the purple yam using fork or back of a spoon , add few tablespoon of evaporated milk while mashing add a total of 1/4 cup  evaporated milk to get a smooth consistency. this may yield 1.5 to 2 cups of purple yam
2. sift all the dry ingredients set aside.
3. preheat oven to 325f
4. in a mixing bowl  beat the egg whites with whisk attachment (speed 8) until foamy then start adding the sugar and continue beating until soft peaks form, transfer to a clean dry bowl.
5. using the mixing bowl (no need to wash) add the egg yolks and oil, beat (speed 6) until smooth start adding the purple yam, then the dry ingredients alternately with milk. finally add some coloring.
6. fold in the egg white mixture , do not stir use folding method until everything is well incorporated pour into chiffon baking pans bake for 325F 1 hour, remove from heat and invert the pan let it cool.