Infuse the sugar with lemon zest: In a medium heatproof bowl, combine the sugar and lemon zest. Use your fingertips to rub the zest into the sugar until fragrant and slightly moist.
Whisk the base: Add the egg yolks and whole eggs to the bowl and whisk until well combined and slightly thickened. Then whisk in the lemon juice until smooth.
Cook the curd: Place the bowl over a saucepan with gently simmering water (double boiler method). Cook the mixture, whisking constantly, until it thickens enough to coat the back of a spoon, about 8–10 minutes, or until it reaches 170°F (77°C).
Add the butter: Remove the bowl from heat and immediately whisk in the butter, until completely melted and the curd is glossy and smooth.
Strain for silkiness: Pour the curd through a fine-mesh sieve into a clean bowl or jar to remove any bits of cooked egg or zest.
Cool and chill: Press a piece of plastic wrap directly onto the surface to prevent a skin from forming. Let cool to room temperature, then refrigerate for at least 2 hours before using.
