Lemon Curd
Lemon curd is an egg yolk-thickened cooked emulsion, and it requires patience to avoid curdling. Yolks, sugar, and lemon juice are combined and cooked over indirect heat, either a double boiler or a heatproof bowl over simmering water, while stirring constantly. The yolks begin thickening the mixture at around 70 degrees Celsius. Cold butter is whisked in off the heat once the curd has thickened enough to coat a spoon and hold a line. The butter adds richness, smoothness, and gloss, and its fat disrupts the egg protein network enough to produce a silky texture rather than a gelatinous one. The curd thickens further as it cools. Direct heat is possible but less forgiving; a double boiler gives more control and more time to recognize when the curd is approaching the right consistency. Overcooking produces a grainy, scrambled curd that cannot be saved. Strain the finished curd through a fine mesh sieve and press plastic wrap directly onto the surface before refrigerating to prevent a skin from forming.
Instructions
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Whisk egg yolks, sugar, lemon juice, and lemon zest together in a medium saucepan. Set over medium-low heat.
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Cook, stirring constantly with a wooden spoon or silicone spatula, until the mixture thickens enough to coat the back of the spoon, about 8 to 10 minutes. It should register 170°F on an instant-read thermometer. Do not let it boil or the yolks will curdle.
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Remove from heat. Add the butter pieces one at a time, stirring after each addition until melted and incorporated. Stir in salt.
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Strain through a fine-mesh sieve into a clean jar or bowl to remove the zest and any bits of cooked egg.
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Press plastic wrap directly onto the surface of the curd to prevent a skin from forming. Refrigerate for at least 2 hours until set. Keeps for up to 2 weeks in the refrigerator.
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