In a large bowl, whisk together eggs, egg yolks, cornstarch, and remaining ½ cup (100 grams) sugar. Gradually add warm lemon mixture to egg yolk mixture, whisking constantly. Return mixture to saucepan, and cook over medium heat, stirring constantly in a figure eight motion with a spatula, until thickened and mixture starts to boil. Cook for 1 minute. Strain through a fine-mesh sieve into a medium bowl. Discard solids. Add butter, 1 cube at a time, stirring until combined after each addition and mixture is smooth. Cover with a piece of plastic wrap, pressing wrap directly onto surface of curd to prevent a skin from forming. Refrigerate until completely cool. (This can be made ahead of time; for a quick chill, place bowl in an ice water bath.)