Mix flour, powdered sugar, and salt. Cut in cold butter until pea-sized crumbs form. Add egg yolk and water, mix until dough comes together. Shape into a disk and refrigerate for 30 minutes.
Roll dough to 1/8-inch thick. Cut six 5-5.5 inch circles, press into 4-inch tart pans. Trim edges with rolling pin, prick bottoms with fork, refrigerate 15-20 minutes.
Preheat oven to 375°F. Line each shell with parchment, fill with pie weights. Bake 15 minutes. Remove weights and bake 8-10 minutes until golden. Cool completely.
Whisk egg yolks, sugar, and cornstarch until pale. Gradually whisk in milk. Cook over medium heat, whisking constantly, 5-8 minutes until thickened. Remove from heat, stir in butter and vanilla. Strain and chill with plastic wrap on surface for 2+ hours.
Peel peaches if desired. Slice into 1/4-inch thick crescents. Set aside.
Heat apricot jam and water together until smooth and fluid. Strain if desired and keep warm.
Stir chilled pastry cream. Fill each shell two-thirds full, spread evenly. Arrange peach slices in a fan pattern over cream. Brush with warm apricot glaze. Chill 20-30 minutes before serving.