Preheat oven to 375°F. Line a 9x13-inch baking pan with a parchment paper sling.
Whisk flour, sugar, baking powder, and salt together in a large bowl. Cut in butter using a pastry cutter. Beat egg and vanilla in a small bowl, then add to the dry ingredients, stirring until evenly moistened. The mixture will be fairly dry but clump together if you squeeze a handful of it in your fist.
3 cups (423g) all-purpose flour, 1 cup (200g) granulated sugar, 1 teaspoon baking powder, ½ teaspoon table salt, 1 cup salted butter, 1 large egg, 2 teaspoons vanilla extract
Press half of the dough into the prepared baking dish, reserving about 2 cups for topping the bars.
Combine peaches, sugar, cornstarch, cinnamon, lemon juice, and almond extract in a large bowl. Toss to combine, then spread in an even layer over the bottom crust. Sprinkle the remaining dough over the top of the peaches to form the top crust.
5 cups peaches, ½ cup (100g) granulated sugar, 1 tablespoon cornstarch, ½ teaspoon ground cinnamon, 1 Tablespoon lemon juice, 1 teaspoon almond extract
Bake for 35 to 45 minutes until to top crust is a light golden brown and the center is set. Remove from oven and cool completely.
Whisk glaze ingredients together in a bowl until smooth. Drizzle over the bars, then let the glaze set up for 30 minutes. Slice into squares and serve.
1 cup (120g) powdered sugar, 1 tablespoon milk, ½ teaspoon almond extract