Set a fine mesh sieve over a bowl and slice the butter into pats. Set aside.

Pour the sugar into a pot then zest the lemons over it. Rub the zest between your fingers into the sugar as much as you can, until it becomes like wet sand. This releases the oils for better flavor.

Juice the lemons and add the juice to the pot.

In a separate bowl, beat the eggs with a pinch of fine sea salt using a fork. Do your best to break up all the egg white bits.

Set over medium heat. Cook, stirring very frequently and keeping a close eye on the texture of the curd: at 155 F the curd will start to thicken. Whisk or stir well until it fully thickens and reaches 165 F. Take it off the heat.

Pour the curd through the fine mesh sieve into the bowl. Stir the curd through the sieve to get all of it out, pressing on the zest bits to extract the flavor. Scrape the underside of the sieve to capture all of the curd.

Add the butter pats to the bowl and stir until they fully melt. Let the curd come to room temperature then seal and set in the fridge to chill for a few hours before using.

The curd keeps in the fridge, sealed for about 2 weeks.