Preheat oven to 375 degrees Fahrenheit. Lightly grease a 9x13-inch baking dish.
FOR THE CRUST/TOPPING: In a medium bowl, whisk together the flour, sugar, baking powder, salt, and cinnamon. Use a fork or pastry blender to cut in the butter until mixture resembles coarse sand. Add the egg and mix with hands until dough starts to hold together, but is still crumbly. Press 2/3 of the mixture onto the bottom of the prepared baking dish. Save the other 1/3 for later.
FOR THE PEACHES: In a medium bowl, whisk the sugar, cornstarch, lemon juice, and cinnamon. Add the diced peaches and toss to combine. Evenly distribute the peaches over the bottom crust. Crumble the remaining crust mixture over the peach layer. (I like to squeeze/clump it together in my hands first, for bigger chunks.)
Bake for 45 to 50 minutes or until lightly golden brown on top and bubbling in the center. Cool completely.
FOR THE GLAZE: Whisk ingredients together in a small bowl, adding milk a little at a time until desired consistency is reached. Drizzle over cooled bars. Cut and serve. Keep covered and store at room temperature.