Preheat the oven to 300°F (150°C).
In a medium saucepan, bring the cream to a simmer on low heat. Don’t boil. Turn the heat off.
In medium bowl, whisk together egg yolks, 1/3 cup of sugar and vanilla extract.
To temper the egg yolk mixture, pour about 1/3 cup of warm cream while continuously stirring the egg yolk mixture. Add remaining cream, salt and lemon zest and mix well. Strain the mixture through a fine-mesh sieve.
Place 4 ramekins in a deep baking pan and divide the custard into ramekins. Pour hot water into the baking sheet to reach halfway up the side of ramekins. Bake for about 30 minutes, or until the custard is set.
Take the custards out of water bath and let them cool to room temperature. Then refrigerate loosely covered with plastic wrap at least 3 hours or overnight.
Right before serving, heat the broiler. Evenly sprinkle 1 tablespoon of sugar on each custard and broil for 2-3 minutes until sugar is melted and caramelized. Keep a close eye to prevent burning.