Set a heatproof bowl over a saucepan of gently simmering water, making sure the bowl does not touch the water.
In the bowl, whisk eggs, egg yolks, sugar, lemon juice, lemon zest, and salt until smooth.
Cook over medium-low heat, whisking constantly, for 8–12 minutes until curd thickens enough to coat the back of a spoon, or reaches 170–175°F.
Remove from heat and whisk in butter one tablespoon at a time until fully melted and smooth.
Optional: Strain through a fine-mesh sieve for extra smooth texture.
Press plastic wrap directly onto the surface and cool. Refrigerate at least 2 hours before using.