Using a stand mixer fitted with the paddle attachment (or a large mixing bowl with a hand mixer), combine the butter, sugar, egg yolks and beat on medium-high speed for 2 minutes. Add the lemon juice and zest and beat until just combined.
Add the mixture to a medium saucepan and cook over medium-low heat until smooth. Increase to medium heat and cook, stirring constantly, until the mixture is thick and yellow in color, about 10 minutes.
Remove from the heat and let cool. Pour into a bowl and press plastic wrap directly on surface to prevent skin from forming. Chill in refrigerator for up to 3 days.
Notes
Always use ripe fresh lemons to create this recipe from scratch!
When whisking the mixture on the stove top, be sure you’re stirring constantly to avoid the bottom of the pan scorching the mixture. You will also want to avoid finding bits of egg in your mixture.
Use a double boiler as extra insurance not to burn the mixture.
Press a piece of plastic wrap directly onto the surface when chilling to prevent skin from forming on top of the curd.
Consider creating another types of fruit curd using other fruits. Experiment with orange curd, lime curd and cranberry curd.
If you find a clumpy mixture after refrigeration, try pushing it through a fine mesh strainer to achieve a more creamy texture.
To thicken your curd if it seems too thin, create a cornstarch slurry by combining 2 tablespoons cornstarch and 2 tablespoons water in a small bowl. Add it to the mixture and whisk until cornstarch is fully dissolved. Let simmer for 3 minutes.
After pulling the curd out of the fridge, you may encounter a film on the top. Simply scrape the surface of the curd with a spoon to remove the film.