Preheat oven to 350 F. Grease a 9x 13 baking pan. In a large bowl, whisk together flour, sugar, baking powder, baking soda, cinnamon and salt. Stir in melted butter, sour cream and eggs. Pour and scrape the batter into the prepared pan. Lay the diced apples on top of the batter, spacing them evenly across the entire surface.
Make the crumb topping: Combine flour, brown sugar, melted butter, walnuts salt and cinnamon in a medium bowl. Stir until all the ingredients are incorporated. Drop dollops of the crumb topping all over the apples, distributing them evenly.
Bake the cake for 35-40 minutes, or until the cake is set and a toothpick inserted at the center of the cake comes out clean. Let the cake cool slightly, then drizzle the brown sugar glaze all over the cake.
In a medium saucepan bring the brown sugar and water to boil. Let it simmer until the sugar is dissolved and the mixture has thickened slightly. Stir in the vanilla extract. Drizzle over the cake.
Recipe adapted from Spicy Southern Kitchen.