Combine egg yolks, sugar, lemon juice, and salt in a non-reactive medium-sized saucepan (see note). Whisk until well-combined.

Transfer to stovetop over low/medium-low heat and add butter and lemon zest.

Cook, stirring constantly (do not let the curd boil) until thickened and it coats the back of a spoon (see video below if a visual is helpful to you). I also like to use an instant read thermometer to check that the curd reaches 160F (to ensure eggs are cooked all the way through).

Remove from heat and pour through a fine mesh strainer into a heat-proof bowl.

Cover with plastic wrap touching the surface of the lemon curd to avoid a skin forming. Allow to cool to room temperature for 20-30 minutes then transfer to the refrigerator or divide into serving jars. Chill for several hours then use as desired.