Preheat the oven to 350°F, grease an 8x8-inch baking pan and line with parchment paper. Then set aside.
To make the crumb topping: In a mixing bowl, add together 1 ¼ cup all-purpose flour, ⅓ cup brown sugar, 2 tbsp granulated sugar, 1 tbsp cinnamon, and ¼ tsp salt. Stir everything together and add ½ cup melted butter. Stir again until you have the crumbs.
To make the cake: In a mixing bowl stir together 1 cup all-purpose flour, ½ tsp baking soda, ½ tsp baking powder, and ¼ tsp salt. Then set aside.
In a separate large bowl, add ½ cup room temperature butter and ⅔ cup granulated sugar. Using a hand mixer or a spatula, cream it for a couple of minutes, until light and fluffy.
Add 2 eggs and 1 tsp vanilla extract. Mix again.
Add ⅔ cup sour cream and 1 tbsp milk, mix until well incorporated.
Add dry ingredients, and using a spatula, fold it in until combined but don’t overmix.
In a separate mixing bowl, add peeled, cored, and diced apples, 1 tbsp granulated sugar, and 1 tsp cinnamon and mix well.
Transfer the batter into a pre-greased baking pan and spread it evenly.
Add the mixed apples evenly on top of the cake batter.
Top with the crumb topping.
Bake at 350°F for 50-60 minutes or until a toothpick inserted in the center comes out clean.
When done, take it out of the oven and let it cool off for 10 minutes, then transfer to a wire rack. Drizzle with vanilla glaze before serving.