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1
Pour water to a depth of about 2 inches into a saucepan, place over medium heat, and bring to a simmer.
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2
Combine the lemon juice, whole eggs, yolk, sugar, and salt in a stainless steel bowl that will rest securely in the rim of a saucepan over, not touching, the water. (Never let the egg yolks and sugar sit together for more than a moment without stirring; the sugar will cook the yolks and turn them granular.) Place the bowl over the saucepan and continue to whisk until the mixture becomes very thick and registers 180u00b0 F on a thermometer. This will take 10 to 12 minutes. If you don't have or trust your thermometer, don't worry. It should thicken to the point that your whisk leaves a trail through the curd.
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3
Remove the bowl from over the water and let cool to 140u00b0 F, stirring from time to time to release the heat.
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4
Meanwhile, cut butter into 1-tablespoon (15-ml) pieces. When the cream is ready, leave it in the bowl if using an immersion blender, or pour it into a countertop blender. With the blender running, add the butter 1 tablespoon at a time, blending after each addition until incorporated before adding the next piece. The cream will be pale yellow and opaque and quite thick.
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5
You can use the cream immediately, or pour it into a storage container with a tight-fitting lid and refrigerate for up to 5 days. To use after refrigeration, if necessary, gently heat in a stainless steel bowl set over simmering water until it has softened, whisking constantly.
