Preheat oven to 400 degrees F. Grease a 17 x 12-inch sheet pan with non-stick spray. Set aside.
FOR CAKE: In a large bowl, combine flour, baking soda, both sugars, cinnamon and nutmeg. Use a whisk to combine and sift together. Set aside.
In a large saucepan, over medium heat, add butter and water. Stir until butter melts. Bring to a boil, once it starts to boil remove from heat and add flour mixture. Stir to combine. Then stir in buttermilk, eggs and vanilla extract. Fold in chopped apples.
Spread cake batter evenly into the prepared sheet pan.
Bake 18 to 20 minutes, or until toothpick inserted in center comes out clean. Remove from oven and let cool slightly while you make the glaze.
For GLAZE: Place butter and milk in a medium microwave-safe bowl. Heat until butter melts. Using a whisk, stir in powdered sugar, caramel syrup and vanilla. Stir until smooth and combined.
Pour glaze evenly over warm cake. Let stand 20 minutes or so before serving. Slice into squares and enjoy!