Key Lime Curd

In the top bowl of a double boiler (or a heatproof bowl that fits snugly over a saucepan), whisk together the key lime juice, sugar, whole eggs, yolks, zest, and salt until the sugar shows signs of dissolving and the mixture is uniformly combined. There should be no unincorporated yolk streaks. Do not begin heating yet — the pre-mix ensures even cooking and prevents the eggs on the hot bowl bottom from scrambling before the sugar dissolves.

Place the bowl over a pot with 3–4 cm of barely simmering water — the bowl must not touch the water surface. Cook over medium-low heat, stirring constantly with a heatproof spatula or whisk, making sure to scrape the sides and bottom of the bowl. The curd will thicken gradually over 10–15 minutes. It is done when it coats the back of a spoon and holds a line drawn through it. Do not rush this with higher heat — scrambled eggs are irreversible.

Remove the bowl from the heat. Add the cold butter cubes all at once and stir in vigorous circles until every cube has melted completely into the curd. The cold butter lowers the temperature, stops the cooking, and emulsifies into the curd to create a glossy, rich texture. Strain the finished curd through a fine-mesh strainer into a clean bowl to remove the zest and any stray egg solids. Press plastic wrap directly onto the surface of the curd (touching it completely) to prevent a skin forming. Refrigerate until cold and set.