Prepare the Ingredients:

1. Preheat your oven to 350°F (175°C). Grease a jelly roll pan or a 16.75×12-inch pan with nonstick spray or butter.
2. Peel, core, and chop the Granny Smith apples into small pieces. Set aside.

2. Mix the Dry Ingredients:

1. In a large mixing bowl, whisk together flour, cinnamon, baking soda, salt, granulated sugar, and brown sugar.

3. Prepare the Wet Ingredients:

1. In a medium saucepan, combine melted butter and water. Heat over medium-high heat until the butter is fully melted and the mixture starts to boil.
2. Pour the hot butter mixture into the dry ingredients and mix well until combined.
3. Allow the mixture to cool slightly, then add buttermilk, eggs, and vanilla extract. Stir until smooth.
4. Gently fold in the chopped apples.

4. Bake the Cake:

1. Pour the batter into the prepared pan and spread it evenly.
2. Bake for 17-20 minutes, or until a toothpick inserted into the center comes out clean.
3. Cool the cake in the pan on a wire rack for 15-20 minutes.

5. Make the Glaze:

1. In a heatproof bowl, microwave butter and milk until melted and warm.
2. Gradually whisk in powdered sugar, vanilla extract, and salted caramel sauce until smooth. Adjust consistency with extra powdered sugar or milk as needed.

6. Assemble and Serve:

1. Pour the glaze evenly over the slightly warm cake, spreading it gently with a spatula.
2. Let the glaze set for about 15 minutes before slicing into squares.
3. Serve warm or at room temperature. Optional: garnish with a drizzle of caramel sauce or a sprinkle of cinnamon.