In a medium saucepan, whisk together sugar and cornstarch. Add egg yolks and egg; whisk until smooth. Whisk in lemonzest and juice, 5 tablespoons (75 grams) water, and salt until smooth. Cook over medium-low heat, whisking constantly, until mixture is thickened and bubbly, about 7 minutes.

Remove from heat, and strain through a fine-mesh sieve into a heatproof bowl. Add butter, and stir until completely incorporated. Cover with plastic wrap, pressing wrap directly onto surface of curd to prevent a skin from forming. Refrigerate until completely cooled, at least 1 hour.