Prepare: place a fine mesh sieve over a medium-size glass bowl then set aside.
Whisk: to another medium-size glass bowl, preferably one with a lip for easy pouring, add the whole eggs plus egg yolks then vigorously whisk until smooth. I like to use a milk frother to help get the eggs very smooth.
Heat: to a medium-size, heavy-bottom saucepan add the lemon juice, sugar, and salt. Turn the heat to medium then whisk to combine the ingredients. Cook while slowly whisking until the sugar has dissolved, 2-3 minutes.
Temper: remove the pan from the heat then slowly stream ~3/4 cup of the hot lemon-sugar mixture (I use a small ladle) into the bowl with the eggs while whisking vigorously. Note: the process of adding hot liquid to cold eggs is called tempering the eggs and it ensures that they don't scramble when they’re added to the remaining hot liquid in the pan.
Cook: slowly stream the tempered eggs into the saucepan with the remaining lemon-sugar mixture while whisking vigorously. It's ok if the mixture foams while whisking. Place the pan over medium-low heat then whisk continuously until the mixture has thickened, 4-5 minutes — this will happen all of the sudden near the end. Avoid cooking over too high of heat and/or overcooking the lemon curd or it can taste eggy.
Strain: scrape the lemon curd into the fine mesh sieve over the glass bowl with a spatula then press it through, catching any solid, cooked pieces of egg in the sieve.
Cool: add half the cold butter then stir until melted. Add the remaining butter then stir until melted. Add the vanilla extract then stir until combined. Place a piece of saran wrap directly on top of the lemon curd to prevent a skin from forming then refrigerate until completely chilled before using, 2-3 hours. The Lemon Curd will thicken once it's completely cool.
Store: Lemon Curd can be refrigerated for up to 2 weeks, or frozen for up to 3 months. Thaw in the refrigerator.
Notes
Estimated nutritional information is for entire recipe.
You will need to use fresh-squeezed lemon juice for this recipe, vs bottled.
Lemon Curd should be cooked to 175 - 180 degrees as measured by a digital thermometer for optimal thickness/texture.
Suggested uses:
As "pudding" — scoop into a cup then eat with a spoon!