Peach Filling: Combine sugar, cornstarch, and salt in a medium saucepan. Stir in egg yolk and peaches. Cook over medium heat, stirring constantly, until mixture comes to a boil. Cool. Dough: Combine 3/4 cup flour, sugar, undissolved yeast, and salt in mixer bowl. Heat milk, water, and butter until very warm (120° to 130°F). Add to flour mixture and beat 2 minutes at medium speed of electric mixer. Add egg yolks and 1/2 cup flour. Beat at high speed of electric mixer, scraping bowl occasionally. Stir in enough remaining flour to make soft dough. Knead on lightly floured surface, about 8 to 10 minutes. Cover; let rest 10 minutes. Divide dough in half. Roll out each half on a floured surface to fit an ungreased 9 x 9-inch pan. Transfer one layer to the pan. Spread with cooled Peach Filling. Top with remaining dough layer. Seal edges together. Snip surface of dough with scissors to let steam escape. Cover; let rise in warm, draft-free place until doubled in size, about 1 hour. Bake at 375°F for 35 to 40 minutes or until done. Let cool in pan. Drizzle, while warm, with Powdered Sugar Glaze. Cut into squares to serve. Powdered Sugar Glaze: Combine powdered sugar and milk in a small bowl. Stir until smooth.