In a mixing bowl, whisk together the flour, 1/4 cup sugar, and 1/4 teaspoon salt. Cut in the cold butter with a pastry cutter or your fingertips until the mixture looks like coarse crumbs with pea-sized bits of butter.

Stir in the egg yolk, then add ice water 1 tablespoon at a time, tossing gently, just until the dough holds together when pressed. Do not overwork.

Turn the dough onto a lightly floured surface, press into a disk, wrap in plastic, and chill for 30 minutes.

Preheat the oven to 375°F. Lightly grease 8 mini tart pans (about 3–4 inches each).

Divide the chilled dough into 8 equal pieces. Press each piece evenly into the bottom and sides of the tart pans, trimming any excess. Prick the bases with a fork.

Line each shell with a small piece of parchment and fill with pie weights or dried beans. Blind bake for 10–12 minutes, then remove parchment and weights and bake 5–8 minutes more, until lightly golden. Cool completely on a rack.

While the shells cool, make the lemon curd. In a heatproof bowl, whisk together lemon juice, lemon zest, 1/2 cup sugar, 4 egg yolks, and a pinch of salt.

Set the bowl over a saucepan of gently simmering water (double boiler), whisking constantly until the mixture thickens enough to coat the back of a spoon, 6–10 minutes.

Remove from heat and whisk in the 4 tablespoons butter, a few pieces at a time, until smooth and glossy. Strain through a fine mesh sieve into a clean bowl to remove zest and any cooked egg bits.

Let the lemon curd cool slightly, then spoon or pipe it into the cooled tart shells, filling almost to the top. Chill the filled tarts for at least 45–60 minutes to set.

For the lilac meringue, place the egg whites in a clean, grease-free mixing bowl. Add cream of tartar.

Beat the egg whites on medium speed until foamy, then gradually add the 3/4 cup sugar in a slow stream, increasing speed to medium-high.

Continue beating until stiff, glossy peaks form and the sugar is fully dissolved. Beat in lilac syrup or infusion, 1 tablespoon at a time, tasting as you go, and add vanilla if using.

Spoon or pipe the lilac meringue in swirls on top of the chilled lemon tarts, piling it high for a cloud-like effect.

Using a kitchen torch, lightly toast the tips and ridges of the meringue until golden. Alternatively, place the tarts under a preheated broiler for 30–60 seconds, watching constantly, until lightly browned.

Serve the tarts soon after topping with meringue, or chill for up to a few hours before serving.