This simple, classic, easy-to-whip-up lemon curd is a bright foil for rich buttercream in Sasha Piligian's Lemon Chiffon Cake with Blueberry-Coriander Buttercream. Regular lemons can be used in place of Meyer lemons.
Whisk together sugar, lemon zest and juice, eggs, egg yolks, and salt in a heatproof bowl. Place bowl over a saucepan of simmering water over low. Cook, whisking constantly, until mixture thickens, 7 to 9 minutes. Remove from heat; whisk in butter until melted and smooth. Pour through a fine wire-mesh strainer into a bowl; discard solids. Place a piece of plastic wrap directly on surface of curd, and chill until cold, about 2 hours.
Curd can be made up to 2 weeks ahead and refrigerated in an airtight container.