LEMON CURD FILLING: In a medium saucepan, whisk together the eggs, sugar, cornstarch, if using, lemon juice, zest, & salt until smooth. Place the pan over medium-low heat. Stir constantly with a wooden spoon or silicone spatula, making sure to scrape the corners of the pan to prevent scrambling. Cook for 5–8 minutes until the mixture thickens & coats the back of the spoon. It should just begin to bubble. Remove from heat immediately. Stir in the cold, cubed butter one piece at a time until completely melted and smooth. Pour the curd through a fine-mesh strainer into a jar or bowl to remove any bits of zest or egg. Press plastic wrap directly onto the surface of the curd to prevent a skin from forming. Refrigerate for at least 1 hour, or until completely cold and firm.