In a small bowl, whisk together 1/4 cup of the sugar and the lemon zest.
In a small saucepan heat the cream over medium heat until warm. Do not allow cream to boil. Whisk in the sugar-zest mixture, remove the pan from the heat and allow mixture to cool. Transfer to a small bowl, cover with plastic wrap and chill for 2 to 3 hours to allow the lemon flavor to infuse into the cream.
Preheat the oven to 300 degrees F.
In a saucepan, reheat the cream mixture until warm, making sure the mixture does not boil. In a mixing bowl, whisk the egg yolks lightly and then drizzle in the warm cream, whisking to combine. Strain the cream mixture into 2 heart-shaped ramekins or other 6-ounce, shallow ramekins and place in a small baking dish. Add enough hot water to come halfway up the sides of the ramekins. Bake the custards in the middle of the oven until just set, 35 to 40 minutes. Carefully remove the ramekins from the water bath and let cool on wire racks. Chill, uncovered, at least 4 hours or up to overnight.
Preheat the broiler and remove the ramekins from the refrigerator for 15 to 20 minutes before proceeding.
Sprinkle the 1 1/2 teaspoons of sugar evenly over each of the custards. Transfer the custards to a baking sheet and broil about 1-inch from the heat, until the sugar is caramelized, 1 to 3 minutes. Serve immediately.