Preheat your oven to 350°F. Grease a 12X18 inch baking pan or line it with parchment paper.
In a large mixing bowl, whisk together the flour, sugar, baking soda, salt, and cinnamon. Set aside.
In a small saucepan, melt the 1/2 cup butter over medium heat. Add the water and bring to a simmer.
Pour the butter and water mixture over the dry ingredients and stir until combined.
Mix in the sour cream, then the 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 30-35 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.
In a large saucepan, melt the 1 cup butter over medium heat. Add the brown sugar and heavy cream. Bring the mixture to a boil, and let it boil for about 2 minutes, stirring constantly.
Remove from heat and cool slightly (about 2-3 minutes).
Gradually add in the powdered sugar and vanilla extract, using a hand mixer on high, 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. *see note
Pour the icing over the cooled cake, quickly spreading evenly over the surface.*To make sure you don’t end up with clumps of powdered sugar, I highly recommend using a hand mixer instead of just a whisk for this step.*Let the icing set for about 20-30 minutes before serving. This allows it to firm up a bit and makes for easier slicing.