In a bowl, whisk together the eggs, egg yolks, sugar, lemon zest, and salt. Gently whisk in the lemon juice and dissolved cornstarch. Place this mixture in a double boiler over simmering water. Using a flat wooden spoon, stir constantly in a figure eight pattern until the mixture coats the spoon, approximately 10 minutes. When the custard starts to adhere to the back of the spoon, remove the pan from the heat and add the butter.

Once the butter is melted, taste the custard. Add the orange flower water if a more citrus flavor if desired. Transfer the lemon curd to a bowl and set over ice to cool quickly, or in the refrigerator. It will thicken as it cools. When cool, transfer it to a jar, or use the entire amount to make a lemon tart.