Add the sugar and lemon zest to a food processor.

Pulse until the zest is fully incorporated and the sugar is fragrant and light yellow. (This step is optional but enhances the lemon flavor.)

Juice your lemons until you have about ½ cup (minus 1 tablespoon) of juice. Strain to remove any seeds or pulp.

In a bowl, whisk the egg yolks. Optionally, strain them into your cooking pot to remove any non-yolk bits.

Add the lemon-sugar mixture to the egg yolks and whisk until light and well combined.

Slowly whisk in the lemon juice until incorporated.

Place the mixture in a heavy-bottomed saucepan over low heat.

Cook, whisking constantly, until the mixture thickens and can coat the back of a spoon. When you draw a line with your finger on the spoon, it should hold its shape.

Remove the pot from heat.

Add the cold butter cubes and stir until completely melted and the curd is smooth.

Strain the curd through a fine mesh sieve into a bowl or jar to remove any cooked egg bits and zest if desired.

Press a sheet of plastic wrap directly onto the surface of the curd to prevent a skin from forming.

Chill in the refrigerator until completely cool and set.