1. Zest two of the lemons into a medium sized metal bowl and set a mesh strainer over the top.
2. In a medium saucepan, warm the lemon juice, sugar, and butter, stirring occasionally, until the butter is melted.
3. In another bowl, stir together the eggs and yolks.
4. When the lemon juice mixture is warm, starting off gradually, pour it slowly into the eggs, whisking constantly. Scrape the egg mixture back into the saucepan and cook the curd, stirring constantly with a heatproof spatula, until the mixture thickens and starts to coat the spatula. (You may see little bits of cooked egg whites, which is normal.) It will happen relatively quickly – perhaps in less than 2-3 minutes, so watch carefully.)
5. Immediately strain the curd into the zest, pressing the curd through the strainer to get most of it through. Wipe the spatula clean then use it to scrape the curd off the bottom of the strainer and into the bowl.
6. Stir the curd frequently to let the steam and heat out, then continue to let it cool for about 10 minutes. Pour the warm lemon curd into jars, cover, and refrigerate until ready to serve.