In a small bowl whisk together granulated sugar and zest. In a saucepan heat cream over moderate heat until warm (do not boil) and whisk in sugar mixture. Remove pan from heat and let mixture cool. Cover surface of cream with plastic wrap and chill several hours or overnight. Preheat oven to 350 F. In a saucepan reheat cream mixture over moderate heat until warm. In a bowl whisk together egg yolks and add cream mixture in a stream, whisking until combined. Strain custard through a fine sieve into a large measuring cup. Pour custard into eight ?½-cup ramekins and arrange ramekins in a shallow baking pan. Fill baking pan with enough hot water to reach halfway up sides of ramekins. Bake custards in middle of oven until just set, 30 to 35 minutes. Let ramekins cool on a rack and chill, uncovered, at least 4 hours or overnight Preheat broiler. Force brown sugar through a sieve evenly over each custard. Transfer custards to baking pan and fill pan with ice. Broil custards about 1 inch from heat until sugar is caramelized, 1 to 2 minutes. Serves 8.