Preheat the oven to 350 degrees F. Place the cream and sugar in a small pot and set over medium heat. Heat until just below a simmer; do not boil. Remove from heat; whisk in the lemon curd until well combined. In a bowl, whisk the egg yolks and lemon extract until combined. Very slowly whisk in the cream mixture. Place 4, 3/4-cup custard or souffle cups in a baking dish. Divide and pour egg/cream mixture into them. Fill the baking dish with enough hot water to come halfway up the sides of the cups. Bake 30 minutes, until just set still but still jiggly. Remove the cups from pan and cool on rack to room temperature. Cover and refrigerate until cold. Sprinkle the brown sugar evenly over the custards. Broil, about 6-inches from heat, until the sugar melts and bubbles, about 1 to 2 minutes. Serve immediately.