In a large saucepan, combine the sugar, lemon zest, corn starch and salt, whisking to combine. Add in lemon juice and eggs. Bring to a simmer over low heat, mixing with a wooden spoon. Simmer about 4-5 minutes until mixtures thickens and coats the back of a spoon.
Remove from heat and add butter, stirring until melted.
Pass curd through a fine mesh sieve. Store in glass jar for up to a week. Mixture will thicken as it cools.
Optional step, bloom one teaspoon of gelatin in one tablespoon of water while lemon mixture is simmering. Add in gelatin along with butter. This will give the curd a slightly firmer texture and is great if you need it to hold up in baked goods.
