Heat the remaining milk. When it begins to boil, reduce the fire and simmer, stirring frequently till it is reduced to about 2 cups. Add the corn flour mixture, condensed milk and sugar. Heat till the mixture begins to boil. Reduce heat and simmer for 2 minutes. Cool completely and add the cream and essence. Taste and add more sugar if needed (add gelatin if using).
Chill in the refrigerator over night. Next day, beat the mixture till frothy. Place in a flat aluminium container in the freezer. When 1/2 frozen, beat till fluffy and return to freezer. Freeze till firm. During this time, stir with fork a couple of times. When frozen, transfer to a freezer container with lid and store in the freezer. If frozen too hard, soften in refrigerator before serving.