Preheat oven to 325°. In a large saucepan, heat cream until bubbles form around sides of pan; remove from heat. In a large bowl, whisk egg yolks, 1/2 cup sugar and salt until blended but not foamy. Slowly stir in hot cream. Stir in lemon zest and extract., Place five 6-oz. broiler-safe ramekins in a baking pan large enough to hold them without touching. Pour egg mixture into ramekins. Place pan on oven rack; add very hot water to pan to within 1/2 in. of top of ramekins. Bake until a knife inserted in the center comes out clean (centers will still be soft), 45-50 minutes., Immediately remove ramekins from water bath to a wire rack; cool 10 minutes. Refrigerate until cold., To caramelize topping with a kitchen torch, sprinkle custards evenly with remaining sugar. Hold torch flame about 2 in. above custard surface and rotate it slowly until sugar is evenly caramelized. Refrigerate 30-60 minutes before serving., To caramelize topping in a broiler, preheat broiler and place ramekins on a baking sheet; let stand at room temperature 15 minutes. Sprinkle custards evenly with remaining sugar. Broil 3-4 in. from heat until sugar is caramelized, 2-3 minutes. Refrigerate 30-60 minutes before serving.