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.