Whisk together the eggs and sugar in a small saucepan. Mix in lemon juice and heat on medium heat while stirring constantly.
The mixture will begin to thicken after 7-10 minutes. When it becomes opaque and custard-like, stir in the butter until it melts into the curd, one cube at a time.
When all the butter is combined, do a streak test by scraping a spatula across the pan through the curd. If the curd parts on both sides and slowly comes back together after 1 second, the lemon curd is ready.
Push the curd through a fine mesh sieve to get rid of any pieces of cooked egg or lemon seeds that may have gotten in there for a super smooth curd. Transfer to a glass jar to cool completely before storing in the fridge.