Preheat the oven to 325 degrees F and water bath pan.
Carefully separate egg yolks from egg white.
Mix sugar and milk together. Blend in butter and cream cheese and mix until smooth.
Beat egg yolks. Blend in grated ube and ube extract. Mix well until smooth. Blend the 2 mixture together. Set aside.
Sift flour and cornstarch together. Set aside the dry mixture.
Beat egg white until it starts to bubble. Add cream of tartar and continue beating until frothy. Add one tbsp of sugar at a time while continuing to beat. Beat until stiff peak forming a meringue.
Transfer 1/3 of the meringue and 1/3 of the dry mixture in the ube mixture. Fold until well blended. Do the same for the other thirds and the last thirds. Fold until well blended but not over mix.
Pour the batter in the lined 6" round cake pan. Bake on the water bath pan for 15 minutes.
Lower the temperature to 300 degrees F for 45 more minutes. Leave in the oven for another 15 minutes. Transfer to the cooling rack stripping off all the parchment paper.
Enjoy the jiggly cake while warm. Refrigerate for few hours and serve.
