#### Make the crust:
In a bowl or food processor, mix flour and powdered sugar. Add butter and pulse or cut in until mixture resembles coarse crumbs.
Add egg yolk and 1 tbsp cold water; mix just until dough comes together. Add more water if needed.
Shape into a disk, wrap, and chill for 30 minutes.
Roll out dough and press into a 9-inch tart pan. Prick the bottom with a fork.
Bake at 350°F (175°C) for 15–18 minutes or until lightly golden. Let cool.
#### Make the lemon curd:
In a saucepan over medium heat, whisk together lemon juice, zest, sugar, eggs, and salt.
Cook, whisking constantly, until it thickens and coats the back of a spoon (about 8–10 minutes).
Remove from heat and whisk in butter until smooth.
Pour into the cooled crust and smooth the top. Chill until set (about 2–3 hours).
#### Make the meringue:
Beat egg whites with cream of tartar until soft peaks form.
Gradually add sugar and beat until stiff, glossy peaks form.
Add vanilla if using.
#### Assemble:
Spoon or pipe meringue over the chilled tart, making swirls or peaks.
Toast with a kitchen torch, or broil for 1–2 minutes until golden (watch closely!).