1 stick butter, cut into 1-inch slices and chilled
Create a water bath by filling a medium saucepan with 1 inch of water and bring to a simmer. In a heatproof bowl, combine the egg yolks and sugar and whisk until smooth, about 1 minute.
Zest and juice the lemons. You should have about 1/3 cup of juice. Add a little water if you need more liquid. Add to egg mixture and whisk until smooth.
Add the bowl on top of your pot of boiling water and whisk until it becomes thickened and light yellow, about 8 minutes. It should coat the back of a spoon. Remove from heat and add butter pieces one at a time. Stir until melted.
Strain lemon curd through a fine mesh strainer (optional) to remove large zest pieces.
Serve warm or refrigerate for up to 2 weeks. If storing, cover the surface with a layer of plastic wrap to prevent drying out.