01 - Preheat oven to 350°F. In a bowl, combine flour, powdered sugar, and salt. Cut in cold butter until mixture resembles coarse crumbs. Add egg yolk and 1 tablespoon cold water; mix until dough just comes together, adding more water if needed. Divide dough into 6 portions. Press each into a 4-inch mini tart pan. Trim edges and prick bottoms with a fork. Freeze for 10 minutes, then bake for 15–18 minutes until golden. Cool completely.
02 - In a saucepan, whisk together eggs, yolks, sugar, lemon juice, zest, and salt. Cook over medium-low heat, stirring constantly, until thickened and coats the back of a spoon (about 6–8 minutes). Remove from heat, whisk in butter pieces until smooth. Pour curd into a bowl, cover surface with plastic wrap, and chill until set (at least 1 hour).
03 - Spoon or pipe lemon curd into cooled tart shells, smoothing the tops.
04 - In a clean bowl, beat egg whites with cream of tartar until soft peaks form. Gradually add sugar, beating to stiff, glossy peaks. Beat in lilac syrup.
05 - Spoon or pipe meringue onto tarts. Use a kitchen torch to lightly brown the meringue, if desired. Garnish with edible lilac blossoms. Serve immediately or refrigerate up to 4 hours.