In a medium bowl, beat the softened butter and sugar with an electric mixer for about 2 minutes, until light and fluffy.
Add the egg and beat until fully combined.
Stir in the lemon juice and lemon zest until blended. The mixture may look slightly curdled at this point, which is normal and smooths out as it cooks.
Pour the mixture into a 1-quart saucepan and cook over medium-low heat, whisking constantly, for 8 to 10 minutes, until the curd coats the back of a spoon and looks smooth and glossy.
Remove the curd from the heat and transfer it to a bowl or jar.
Press a piece of plastic wrap directly onto the surface of the curd and refrigerate for at least 1 hour. The curd thickens more as it chills.
Notes
Cook Low and Slow: Keep the heat at medium-low and don't rush it. High heat scrambles the egg and turns the curd grainy.Whisk the Whole Time: Stir constantly and watch the edges of the pan, where the curd heats fastest and can overcook before the center thickens.Use a Fine Zester: Zest the lemon with a Microplane or other fine grater so the zest blends into the curd instead of leaving noticeable bits.Strain for the Smoothest Curd: Pour the warm curd through a fine-mesh sieve to catch any bits of cooked egg or zest before it chills.If doubling the recipe, use a 2-quart saucepan.