Squeeze the juice from the lemons and pour into a saucepan. Heat over low heat.
Separately, whisk the eggs and sugar until the mixture lightens.
In a small bowl, pour the cornstarch and mix with a little hot lemon juice (this prevents lumps).
Pour the dissolved cornstarch onto the egg mixture while whisking, also adding the remaining hot lemon juice.
Return everything to the saucepan and heat over low heat, whisking constantly for about 4 minutes. The mixture should thicken visibly. Keep whisking to prevent sticking.
Once you have a smooth texture, turn off the heat and add the butter in pieces. Whisk one last time to smooth it out.
Pour into a clean, dry jar and let cool before refrigerating.
Notes
Once cooled to room temperature, I advise you to store the lemon curd in an airtight container (clean and dry) at fridge during 1 to 2 weeks.