Put a large pot of water on to boil for the water bath. Position a rack in the middle of the oven and heat the oven to 325°F. Put eight 6-oz. ramekins in a large roasting pan or baking dish with high sides.
In a small saucepan, combine the lemon zest, juice, and 1/4 cup of the sugar. Simmer until reduced to 1/2 cup, about 15 minutes; set aside. In a medium saucepan, combine the cream, 1/4 cup of the sugar, and the vanilla seeds and pod (if you’re using vanilla extract, don’t add it yet) and bring to just below boiling. Remove from the heat.
In a medium bowl, beat the egg yolks with the remaining 1/4 cup sugar until smooth. Gently whisk a ladleful of the hot cream into the yolks and then whisk the yolk mixture into the saucepan with the rest of the cream. Cook slowly, stirring constantly, until the mixture reaches 170°F on an instant-read thermometer, 3 to 4 minutes. Stir in the reserved lemon syrup and strain immediately through cheesecloth or a fine sieve. If you’re using vanilla extract, stir it in now.
Divide the mixture among the ramekins in the roasting pan. Pull out the oven shelf, put the roasting pan on it (be sure it’s stable), and pour enough boiling water into the pan so that it comes halfway up the sides of the ramekins. Cover the ramekins with a sheet of foil (simply lay the sheet on top, don’t crimp the edges) and bake for 25 to 45 minutes—start checking early—until the custards are set about 1/4 inch in from the sides, the centers respond with a firm jiggle (not a wavelike motion) when you nudge the ramekins, and the centers of the custards register 150° to 155°F on an instant-read thermometer (the hole left by the thermometer will close up as the custards firm). Let the custards cool to room temperature in their water bath. Remove the custards from the bath, cover them with plastic, and refrigerate for at least 8 hours. Garnish with a dollop of sweetened whipped cream and candied zest or flowers, if you like.
Custards may be baked up to two days ahead and refrigerated, covered with plastic.