Zest the lemons, then juice them into a separate bowl. Separate the egg yolks from the whites, placing the yolks in a saucepan.

Whisk the egg yolks and lemon juice together in the saucepan over low heat until well combined.

Add the granulated sugar and lemon zest to the saucepan, continuing to whisk constantly for about 5-7 minutes until the mixture thickens enough to coat the back of a spoon.

Remove the saucepan from the heat.

Add the cold, unsalted butter to the mixture. Whisk vigorously until the butter is fully melted and incorporated, creating a smooth, glossy curd. Strain through a fine-mesh sieve if desired for an extra smooth texture.

Transfer the lemon curd to a clean jar or airtight container. Let it cool completely before refrigerating. It will thicken further as it chills.