In a medium heatproof bowl, add eggs, sugar, lemon zest, and juice and whisk until combined. Place the bowl over a saucepan of simmering water and cook on medium heat. While the mixture is cooking, whisking constantly until thickened. The curd is ready when it can thickly coat the back of a spoon or a whisker. The curd will firm up once it has cooled.
Remove from heat and immediately strain the mixture through a sieve. Add butter and whisk until the butter completely melted and incorporated or the mixture is creamy and smooth.
Cover with a plastic wrap directly on top to prevent a skin from forming and refrigerate for at least 3-4 hours.
Lemon curd can be kept in an airtight container in the fridge for up to 10 days, or can be frozen for up to 2 months.