Wash the lemons very well because you will be using both the zest and the juice. Using a fine grater or a zester, remove only the yellow part of the peel. Avoid the white pith underneath because it can be bitter. Once you have the zest, cut the lemons in half and squeeze out the juice. You should have about one cup of fresh lemon juice. Strain the juice to remove any seeds or pulp.

To prevent the eggs from cooking too quickly, it is best to use a double boiler method. Fill a saucepan with about two inches of water and bring it to a gentle simmer. Place a heatproof bowl over the saucepan, making sure the bottom of the bowl does not touch the water. This will help the curd cook slowly and evenly.

In the heatproof bowl, whisk together the eggs, egg yolks, sugar, lemon juice, and lemon zest. Stir until everything is well combined and the mixture looks smooth. This step helps the sugar dissolve and mixes the eggs evenly into the liquid.

Place the bowl over the simmering water and start whisking slowly. Continue whisking for about 10 to 15 minutes until the mixture thickens. You will know it is ready when the curd coats the back of a spoon and leaves a clear line when you run your finger through it. Keep the heat gentle because high heat can cause the eggs to scramble.

Once the curd thickens, remove the bowl from the heat. Add the softened butter pieces one at a time, stirring until each piece melts completely. The butter adds a silky smooth texture and a rich flavor to your lemon curd.

For a perfectly smooth lemon curd, pour it through a fine sieve into another clean bowl. This step removes any bits of cooked egg or zest, leaving you with a creamy and velvety curd.

While the curd is still warm, use a clean funnel to pour it into your sterilized jars. Leave about half an inch of space at the top. Wipe the rims of the jars with a clean damp cloth to make sure there is no residue. Seal the jars tightly with the lids.

Bring a large pot of water to a gentle boil and carefully lower the sealed jars into the water using tongs or a jar lifter. Make sure the jars are fully covered with water. Let them process for about 15 minutes. This step ensures that your lemon curd is safe for storage and will last longer.

Remove the jars from the hot water and place them on a towel or cooling rack. Let them cool completely without touching them. Once cooled, check the lids to ensure they are properly sealed. You can store sealed jars in a cool, dark place for up to three months. After opening, keep the lemon curd refrigerated and use it within two weeks.