Homemade Lemon Curd (with variations!)

In the bowl of a stand mixer fitted with a paddle, beat the butter on medium speed until creamy, about 1 minute. Add the sugar and salt and mix on medium speed until combined, another minute more. Scrape down the sides of the mixing bowl, and add the egg yolks on low speed. Increase the speed to medium and beat until smooth and light, 3 to 4 minutes. Add the whole egg and mix on low speed until combined, then add the lemon juice and mix on low low speed until combined, scraping down the sides as needed.

Transfer the mixture to a medium, heavy-bottom sauce pan. Cook over medium heat, stirring constantly with a spatula, until the curd becomes very thick, about 10 minutes, or registers 170F [75C] on an instant-read thermometer. The mixture should coat a spatula at this point. Strain the mixture through a fine-mesh sieve, then cover with plastic wrap, making sure the wrap sits directly on top of the curd (this will help keep it from forming a skin). Place in the refrigerator until well chilled. The curd can be stored in the refrigerator in an airtight container for 5 days.