Preheat oven to 350 degrees F. Line a 9x13 pan with parchment and set aside.
Combine flour, graham cracker crumbs, 1/2 cup brown sugar, 3/4 cup melted butter, granulated sugar and cinnamon together in a mixing bowl. Stir until well combined. Reserve 1 cup of the crumb mixture.
Press remaining crumb mixture into bottom of prepared pan. Bake 10 minutes, then set aside.
Meanwhile, toss remaining 1/4 cup brown sugar with apples. Spoon apples over baked crust. Stir remaining 1/4 cup butter into remaining 1 cup crumb mixture until well incorporated. Sprinkle mixture over apples.
Bake 40-45 minutes, until apples are soft. Cool 20 minutes.
Combine glaze ingredients, adding hot water until glaze is pourable. Drizzle over apple bars. Let cool completely to set. Cut into squares and serve. Store leftovers in an airtight container. Refrigerated leftovers will last longer, but it's not necessary.