Start by preheating the oven to 350°F. Grab a 12x18 baking pan and grease it with cooking spray or line it with parchment paper. Next grab a large mixing bowl and add the flour, sugar, baking soda, salt, and cinnamon. Whisk that together until combined and set aside.
Add ½ a cup of butter to a small saucepan and heat it over medium heat until it melts. Once melted, add the water and heat until it comes to a simmer.
Pour the warmed butter and water mixture over the flour mixture and mix until fully incorporated. Then, add the sour cream and mix. Lastly add the eggs and mix everything until combined. Add the chopped apples and fold everything together until combined. Pour the batter into the prepared baking pan and spread it evenly.
Bake the cake for 30-35 minutes or until done. You can test for doneness by sticking a toothpick into the center of the cake. If it comes out clean with a few crumbs, it’s done. Allow the cake to sit in the pan to cool completely. While it cools, start on the frosting.