Preheat oven to 350F (175C) and lightly grease and flour a 9" (23cm) springform pan.
Combine melted butter and sugars in a large bowl and stir until well-combined
Add eggs and vanilla extract, stir well
Stir in sour cream.
In a separate, medium-sized bowl, whisk together flour, cinnamon, baking powder, baking soda, and salt.
Using a spatula, gradually add dry ingredients and wet and stir until just combined– do not over-mix!
Fold in apple pieces and pour batter evenly into prepared pan. Set aside and prepare your streusel.
Prepare the crumble topping by whisking together flour, sugars, cinnamon and salt.
Pour the melted butter into the mixture and, using a fork, stir until crumbles form. Evenly sprinkle streusel over batter.
Transfer pan to oven and bake on 350F (175C) for 45-50 minutes, or until a toothpick inserted in the center comes out clean. Allow to cool before adding vanilla glaze.
If desired, make glaze by whisking together powdered sugar, milk, and vanilla. Start with 2 teaspoons of milk and add a third if it isn’t thin enough to drizzle.
Drizzle over cooled cake. Allow glaze to harden, slice, and enjoy!