In a medium saucepan over medium heat, combine fresh lemon juice, sugar, butter, and a pinch of salt. Stir occasionally until the butter melts and the mixture is warm.

Whisk the egg yolks and cornstarch in a bowl.

Gradually pour some of the warm mixture into the bowl with the egg yolks, whisking continuously to temper the eggs.

Return the mixture back to the saucepan and cook over medium heat, stirring constantly until thickened (about 5–7 minutes).

Remove from heat and stir in the lemon zest. Let it cool for a few minutes, then spoon it into the mini tart shells.

In a clean mixing bowl, whip egg whites and cream of tartar with a mixer on medium speed until soft peaks form.

Gradually add in sugar, one tablespoon at a time, until glossy and stiff peaks form (about 4–5 minutes).

Add vanilla extract and mix until well combined.

Spoon the meringue over each lemon-filled tart, covering the filling completely with peaks and swirls.

Bake in a preheated oven at 350°F (175°C) for 8–10 minutes or until the meringue is golden brown.

Allow the tarts to cool completely before serving.