Preheat oven to 325°F. Begin boiling a large pot of water that will be used in your water bath. Place 6 4 oz. ramekins in a large roasting pan or baking dish with high sides.
In a small saucepan, combine the lemon zest, lemon juice, and 1/4 cup of the sugar. Simmer until reduced to 1/2 cup, about 10-15 minutes; set aside.
Meanwhile, in a large saucepan, combine the cream and 1/4 cup of the sugar, 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. Slowly whisk a ladleful of the hot cream into the yolks to temper then slowly/ gradually whisk the yolk mixture into the saucepan with the rest of the cream. Cook (still off heat) stirring constantly, until the mixture reaches 170°F on an instant-read thermometer, 3 to 4 minutes until its thick.
Stir in the reserved lemon syrup and vanilla extract. Strain immediately through cheesecloth or a fine sieve then divide the mixture evenly between the ramekins. (I strain mine into a very large pouring measuring cup so I can pour directly after straining - you could also use a pitcher.)
Pour enough prepared boiling water into the roasting pan around the ramekins so it reaches halfway up the sides of the ramekins. Lay a long sheet of foil over the ramekins (no need to crimp edges) and bake 30-45 minutes – until the custards are set about ¼ inch from the sides but the center still jiggles. The center of the custards should register 150° to 155°F on an instant-read thermometer.
Remove custards from oven and let them cool IN their water bath to room temperature. Remove from water bath, cover with plastic and refrigerate at least 8 hours, best if 24.
Garnish with whipped cream and plenty of berries.