1-First Step: Prepare the Tart Shell
Beat the softened unsalted butter until creamy. Gradually add powdered sugar and salt, mixing on low speed until combined. Incorporate the egg and vanilla extract, blending thoroughly. Slowly add the all-purpose flour and mix just until the dough forms a soft, crumbly consistency. Shape into a disc, wrap in plastic wrap, and chill in the refrigerator for at least 1 hour or overnight. Before rolling, let the dough rest at room temperature for 10 minutes.

2-Second Step: Roll and Blind Bake the Crust
Roll the dough to about ⅛ inch (3 mm) thickness on a lightly floured surface. Cut it slightly larger than your tart pan diameter to cover sides fully. Press dough gently into the tart pan and trim excess. Prick the bottom with a fork to prevent bubbles. Line with foil or parchment paper and fill with pie weights or dried beans. Bake at 175°C (350°F) for 15 minutes. Remove weights and lining, then bake for an additional 10 minutes or until golden brown. Allow the crust to cool completely before adding filling.

3-Third Step: Prepare the Lemon Filling
Whisk together the whole eggs, egg yolks, granulated sugar, and freshly squeezed lemon juice in a saucepan. Cook over medium heat, stirring constantly, until the mixture thickens to a custard-like consistency about 15 minutes, being careful not to boil. Remove from heat and incorporate the softened butter a few cubes at a time, stirring until fully blended. Press plastic wrap directly on the surface of the lemon curd to prevent a skin from forming. Let cool to room temperature before pouring into the cooled tart shell. Refrigerate for at least 4 hours or overnight to set fully.

4-Fourth Step: Make the Swiss Meringue Topping
Simmer about 1 inch of water in a pot. In a heatproof bowl set over but not touching the simmering water, whisk together egg whites and sugar. Stir continuously until sugar dissolves and mixture is smooth, about 4 minutes. Remove from heat and add vanilla extract. Beat with an electric mixer on high speed until stiff, glossy peaks form this usually takes 5 to 7 minutes.

5-Fifth Step: Assemble and Bake the Tart
Spread or pipe the meringue over the chilled lemon filling, making sure to seal the edges to prevent shrinking. Bake the tart at 175°C (350°F) for 10 to 12 minutes or until the meringue peaks develop a light golden brown hue. Alternatively, use a kitchen torch to toast the meringue for a visually appealing caramelized finish.

6-Final Step: Cool and Serve
Allow the tart to cool completely at room temperature. For best flavor and texture, refrigerate before slicing. Serve chilled or at room temperature. Store leftovers in a sealed container in the refrigerator.