In a large bowl with an electric mixer, beat butter and sugar with an electric mixer until well combined, about 2 minutes. Slowly add the eggs and egg yolks. beat for another minute then mix in the lemon juice and zest. Don't worry if the mixture looks curdled like in the photo, it will smooth out when it cooks like in the photo above.

Transfer the mixture to a heavy based saucepan and cook over low heat until it looks smooth. Increase heat to medium and cook, stirring constantly, until the mixture thickens, about 12-15 minutes. You will know its ready when it will leave a path on the back of a sppon or register 170°F on a candy thermometer. Do not let the mixture boil!