Juice approximately 4 to 5 large lemons over a fine mesh strainer set up over a glass bowl or large measuring cup. Double strain the juice for smoothest lemon curd. Set juice aside.

In a separate glass mixing bowl, combine the eggs, egg yolks and sugar. Whisk vigorously for 3 to 4 minutes, until the mixture is thick and more pale in color. It should ribbon off the whisk. Pour the mixture into a stainless steel pot and add the lemon juice; whisk to combine.

Cook the lemon curd over medium-low heat, stirring constantly along the bottom of the pan. Cook for 4 to 5 minutes, until the curd thickens and starts to bubble lightly. Remove from heat immediately. Add the unsalted butter and vanilla and whisk until the butter is completely melted.

Transfer the curd into a glass bowl or glass jar and cover with plastic wrap to prevent a film from forming over the top. Allow the lemon filling to cool slightly, then place into the refrigerator. Lemon curd can be stored in the fridge for up to 2 weeks.