Preheat oven to 350 degrees. Line the bottom and sides of an 8-inch square baking pan with aluminum foil or parchment, leave an overhang on all of the sides so that you can easily lift the bars out of the pan before cutting. Spray with nonstick spray or grease with butter. Set aside.

To make the crust and topping: In a large bowl, whisk the flour, oats, brown sugar, and cinnamon together until combined.

Cut in the cold butter using a pastry blender or two forks until it reaches coarse, pea-sized crumbs. Remove 3/4 cup of the mixture and put it in a small bowl, set aside.This will be for the streusel topping.

Evenly press the remaining oat mixture into the bottom of the prepared baking pan. Bake for 15 minutes.

While the crust is baking, prepare the filling: Whisk the egg and sugar together until smooth and creamy.

Add the flour and salt. Whisk until combined and then fold in the peaches.

Remove crust from the oven after 15 minutes and pour the filling over hot crust.

Sprinkle with reserved streusel topping mixture.

Bake for 30-32 minutes or until golden brown on top. Allow to cool in the pan on a wire rack for 30 minutes. Transfer to the refrigerator and allow to chill for 2 hours. This will help the bars set and be easier to cut.

Once chilled, lift the foil out of the pan using the overhang on the sides. Remove parchment paper. Place baked bars on serving plate.

Prepare the glaze: In a small bowl, whisk together the powedered sugar, milk, and vanilla extract together until smooth. Add another Tablespoon of milk to thin out if needed. Drizzle over each square.