01 - Preheat oven to 350°F. Combine flour, powdered sugar, and salt in a bowl. Cut in cold butter until mixture resembles coarse crumbs. Add egg yolk and 1 tbsp cold water; mix until dough just comes together, adding more water if needed. Divide dough into 6 portions and 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 - Whisk together eggs, yolks, sugar, lemon juice, zest, and salt in a saucepan. Cook over medium-low heat, stirring constantly, until thickened and coats the back of a spoon, about 6-8 minutes. Remove from heat and whisk in butter pieces until smooth. Pour curd into a bowl, cover surface with plastic wrap, and chill until set for at least 1 hour.
03 - Spoon or pipe lemon curd into cooled tart shells, smoothing the tops evenly.
04 - Beat egg whites with cream of tartar in a clean bowl until soft peaks form. Gradually add sugar, beating to stiff, glossy peaks. Beat in lilac syrup until fully incorporated.
05 - Spoon or pipe meringue onto filled tarts. Use a kitchen torch to lightly brown the meringue if desired. Garnish with edible lilac blossoms and serve immediately or refrigerate up to 4 hours.