Combine the tofu, cream cheese, sugar and lemon zest in the bowl of a food processor. Process until creamy and perfectly smooth, then add the lemon juice and pulse to combine, scraping the bowl down several times.
Divide the mixture among six wide, shallow heat-proof dishes or bowls. Smooth the tops as flat as possible and chill for at least two hours.
To caramelize, use a brulee torch or preheat broiler until blazing hot. Sprinkle a thin layer of sugar evenly over the surface of the custard. Set the dishes on an upside-down baking dish so the surface are no more than one inch away from the heat. Watch carefully as the sugar melts and then caramelizes; it goes fast once it starts. If you're using a torch, move the torch back and forth until the sugar melts and browns. Let stand a moment, then serve.