Sift flour, cornstarch, baking powder and sugar in a large bowl,make a well in the center and add cooking oil, egg yolks,ube mixture and purple yam flavor and color, using your hand mixer or wooden spoon, mix until smooth but do not over beat. Set aside.
Put egg whites and cream of tartar in the bowl of your mixer until frothy then gradually add sugar and beat until stiff and glossy.
Gently fold egg whites into flour mixture-taking care not to knock the air out. Gently lift and fold until the two mixtures are well-blended-you should have a uniform color (no white streaks).
Divide and pour batter into 2 6-inch parchment paper-lined pan and bake at 325F degrees preheated oven for about 35 minutes.. or
Cover baking pans with aluminum foil and steam for about half an hour (cake is done when a toothpick inserted in the center comes out clean or dry). Transfer to a wire rack, let cool 5 minutes then run a knife around the edges of the pan and invert cakes to cool.
For the Filling/Frosting:
Beat cream cheese and 3 tablespoons of sugar until smooth. Transfer to a small bowl and set aside. In the same bowl, put all-purpose cream and beat gradually adding 4 tablespoons of sugar until stiff.
Fold cream cheese mixture to the whipped cream until well-blended.
Make sure that cakes have cooled completely. Slice each cake horizontally in half. You will have a total of 4 cake disks. Crumble one part of the cake and set aside, you will use this to garnish sides and top of cake.
Lay one layer of cake on a cake stand, spread some filling/frosting on top and lay another cake on top, repeat process until you used all 3 cake discs.