1-First Step: Set up the double boiler Fill the bottom of a double boiler with 1 to 2 inches of water. Bring it to a simmer over high heat, then reduce the heat to low. The water should stay gently steaming, not boiling hard. If you do not have a double boiler, use a heatproof glass bowl set over a pot of simmering water, making sure the bowl does not touch the water.

2-Second Step: Combine the curd base In the top part of the double boiler, whisk together the egg yolks, sugar, lemon zest, lemon juice, and salt. Start whisking right away so the sugar begins dissolving and the eggs stay in motion. A silicone whisk works especially well here because it is gentle on your bowl and easy to clean.

3-Third Step: Whisk until thickened Keep whisking constantly for about 10 minutes, or until the mixture thickens to a hollandaise-like texture. If you are using a thermometer, aim for 170 degrees Fahrenheit. The curd should coat the back of a spoon and look smooth and glossy.

4-Fourth Step: Add the butter Once the curd has thickened, remove it from the heat. Whisk in the softened butter, one piece at a time, until each piece melts into the mixture. This step gives the curd its silky finish and rich flavor. The texture should become smooth and shiny.

5-Final Step: Cool and chill Pour the finished lemon curd into a jar or bowl. Press plastic wrap directly onto the surface so a skin does not form. Let it cool, then refrigerate until fully chilled. Total time, including cooling, is about 1 hour and 15 minutes.