1. Pour soy milk into a medium-sized saucepan. Add salt, sugar, and vanilla sugar. Heat over medium heat; do not stir!
2. Meanwhile, in a separate bowl, combine whipped topping, egg yolks, and cornstarch. When the milk comes to a boil, remove ¼ cup of heated milk, and quickly whisk it into the egg mixture to temper the eggs. Pour the mixture back into the saucepan and stir constantly. Cook for one minute, until custard thickens.
3. Remove from heat and stir in margarine and vanilla extract. Transfer to a medium-sized bowl, and cover with plastic wrap to prevent a skin from forming. Place in refrigerator until cake is ready.