Prepare a double boiler by filling a pot with water and bringing it to a simmer.
In a heatproof bowl, whisk together the sugar and eggs until well combined.
Add the lemon juice, zest, and a pinch of salt to the egg mixture.
Place the bowl over the simmering water, ensuring it doesn't touch the water.
Whisk continuously until the mixture begins to thicken, about 10 minutes.
Remove the bowl from heat and stir in the cubed butter until melted and smooth.
Pass the curd through a fine-mesh sieve to remove any lumps.
Let it cool to room temperature, then transfer to jars.
Refrigerate for at least 2 hours before using.
Enjoy your homemade lemon curd on toast, pastries, or as a filling!