This zesty curd is the ideal filling for sandwiching between layers of luscious Lemon Cake.
Whisk egg yolks, eggs, sugar and lemon juice, and zest in a medium heatproof bowl. Place over a pot of simmering water; whisk until thickened, about 5 minutes. Strain into a small bowl; add butter, and whisk until smooth. Press a piece of plastic wrap directly on surface; refrigerate until cold.