• Preheat your oven to 350°F. Grease a 12x17 or 12x18-inch baking pan or line it with parchment paper.
• Chop 3 cups chopped and peeled apples and set aside.
• In a large mixing bowl, whisk together the 2 cups all-purpose flour, 2 cups granulated sugar, ½ teaspoon baking soda, ½ teaspoon salt, and 2 teaspoons ground cinnamon. Set aside.
• In a small saucepan, melt the ½ cup unsalted butter over medium heat. Add 1 cup water and bring to a simmer.
• Pour the butter and water mixture over the dry ingredients and stir until combined.
• Mix in the ½ cup sour cream, then the 2 large eggs, one at a time, ensuring each is fully incorporated.
• Fold in the chopped apples.
• Pour the batter into the prepared pan and spread it out evenly.
• Bake for about 30 to 35 minutes or until a toothpick inserted into the center comes out clean or with just a few crumbs.
• Let the cake cool in the pan for about 1 hour before icing.

Icing

• In a large saucepan, melt the 1 cup unsalted butter over medium heat. Add the 2 cups light brown sugar and ¾ cup heavy cream. Bring the mixture to a rolling boil for 2 minutes, stirring constantly.
• Remove from heat and whisk in the 3½ cups powdered sugar and 2 teaspoons vanilla extract, until the icing is smooth and creamy. If the icing seems too thick, you can add a little more heavy cream, a tablespoon at a time, to reach the desired consistency.
• Quickly pour the icing over the cooled cake, spreading evenly. The icing will begin to set very fast and be hard to spread and work with as it cools.
• Let the cake set for 20 minutes after icing before slicing and serving.