Step 1Zest the lemons into a stainless steel bowl or the top of a double boiler. Squeeze the juice from the lemons into the bowl. Add the sugar and eggs and beat until blended. Add the butter and set the bowl or the pan over, but not touching, simmering water.
Step 2Cook, stirring almost constantly and scraping down the sides of the bowl, until the mixture thickens enough to heavily coat the back of a wooden spoon, 15 minutes or so. Strain through a fine sieve into a clean bowl. Use immediately or cover with a sheet of plastic wrap pressed onto the surface to prevent a film from forming and refrigerate until needed.