In a saucepan, whisk together the lemon juice, lemon zest, sugar, and eggs. Cook over medium heat, stirring constantly, until the mixture thickens.
Remove from heat and stir in the butter until melted and smooth. Transfer the lemon curd to a bowl and let it cool completely.
5. Assembling the Cupcakes
Hollowing Out Cupcakes
Using a knife or cupcake corer, carefully remove the center of each cupcake to create a hollow.
Filling with Lemon Curd
Fill each hollowed-out cupcake with a spoonful of lemon curd, making sure not to overfill.
Piping Meringue on Top
In a clean mixing bowl, beat the egg whites and cream of tartar until soft peaks form. Gradually add the sugar and vanilla extract, beating until stiff peaks form.
Transfer the meringue to a piping bag fitted with a star tip. Pipe the meringue onto the filled cupcakes in swirls or peaks.
6. Baking the Cupcakes
Preheat the oven and bake the cupcakes until the meringue is lightly golden brown.
7. Cooling and Decorating
Allow the cupcakes to cool completely before serving. You can garnish them with additional lemon zest or small lemon slices for a decorative touch.
