1. In a small saucepan, combine the sugar and cornstarch. Slowly pour in the soy milk and lemon juice while whisking vigorously. Immediately turn the heat on to medium. If you wait too long, the mixture will curdle and separate.
2. Continue to whisk occasionally until mixture comes to a full boil. Liquids should feel significantly thickened. Turn off the heat, whisk in the zest and vanilla, and let cool to room temperature.
3. Place a piece of plastic wrap directly on top of the surface, and thoroughly chill before proceeding; about one hour. Once completely cool, fold in the “sour cream” or tofu, and pipe or spoon the mousse into individual cups. Keep refrigerated until ready to serve.