Add the flour, 2 tablespoons sugar, and salt to a mixing bowl and whisk to combine.
Add the cold cubed butter and cut it into the flour with a pastry cutter or your fingers until the mixture resembles coarse crumbs with pea sized bits of butter.
Whisk the egg yolk with the ice water, drizzle over the flour mixture, and stir until the dough just comes together.
Turn the dough onto a lightly floured surface, gently press into a disk, wrap, and chill for 30 minutes.
Preheat the oven to 375°F and lightly grease a 12 cup muffin tin.
Roll the chilled dough to about 1/8 inch thick, then cut 12 circles large enough to line the muffin cups.
Gently press the dough circles into the muffin cups without stretching, trimming any excess from the top.
Prick the bottoms of the shells with a fork and chill the pan for 10 to 15 minutes.
Line each shell with a small piece of parchment and fill with pie weights or dried beans.
Bake the shells for 10 minutes, then carefully remove the parchment and weights and bake for 5 to 7 minutes more, until lightly golden.
Remove the pan from the oven and let the tart shells cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.
Fill a saucepan with 1 to 2 inches of water and bring to a gentle simmer, then reduce the heat to low.
In a heatproof bowl, whisk the whole eggs, egg yolks, 1/2 cup sugar, lemon juice, and lemon zest until smooth.
Set the bowl over the simmering water, making sure the bottom does not touch the water, and cook, stirring constantly, until the mixture thickens and coats the back of a spoon.
Remove the bowl from the heat and immediately whisk in the room temperature butter, a few pieces at a time, until the curd is smooth and glossy.
Strain the lemon curd through a fine mesh sieve into a clean bowl, then let it cool for 10 minutes at room temperature.
Spoon or pipe the lemon curd into the cooled tart shells, filling almost to the top, and chill for at least 30 minutes until set.
When ready to make the meringue, preheat the broiler or have a kitchen torch ready.
Add the egg whites and cream of tartar to a clean mixing bowl and beat on medium speed until frothy.
Gradually add 1/3 cup sugar in a slow stream while beating, then increase to medium high speed and whip until stiff, glossy peaks form.
Add the lilac extract and food coloring, if using, and beat briefly just until evenly combined.
Spoon or pipe the lilac meringue over the chilled lemon tarts, piling it slightly higher in the centers and creating swirls and peaks.
Lightly toast the meringue with a kitchen torch until the tips are golden, or place the tarts under the broiler for 1 to 2 minutes, watching closely to avoid burning.
Let the tarts stand for 5 to 10 minutes so the meringue sets slightly, then serve or chill until ready to serve.