In a large bowl, combine the flour, cinnamon, baking soda, salt, granulated sugar, and brown sugar.
In a medium saucepan over medium-high heat, combine the butter with 1 cup water and heat until the butter melts. Bring to a boil. Once the mixture is boiling, remove from heat and stir into the flour mixture. Let cool slightly then add add in the buttermilk, eggs, and vanilla. Gently stir in the apples.
Pour the batter into the prepared pan, spreading evenly. Bake for 20 minutes if using a jelly roll pan, and about 17 minutes if using a 16.75 by 12-inch pan, or until a toothpick inserted comes out clean. Let cool in the pan on a wire rack for about 15 to 20 minutes.
Make the glaze:
Meanwhile, combine the butter and milk in a large heat-proof bowl. Microwave until the butter is melted. Gradually stir in the powdered sugar until combined. Stir in the vanilla and caramel until combined. If the glaze is too thin, add more powdered sugar to thicken. Pour evenly over the slightly warm cake. Feel free to use less glaze for less sweetness. Let set for about 15 minutes before slicing and serving.