• 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
  ¾ cup unsalted butter, ½ cup granulated sugar, ¼ cup light brown sugar
• Add eggs and vanilla extract, stir well
  2 large eggs, 1 ½ teaspoon vanilla extract
• Stir in sour cream.
  ½ cup sour cream
• In a separate, medium-sized bowl, whisk together flour, cinnamon, baking powder, baking soda, and salt.
  
  1 ¼ cups all-purpose flour, 1 teaspoon ground cinnamon, ¾ teaspoon baking powder, ¾ teaspoon baking soda, 1 teaspoon 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.
  1 ½ cups diced, peeled apples

Streusel

• Prepare the crumble topping by whisking together flour, sugars, cinnamon and salt.
  1 cup all-purpose flour, ¾ cup light brown sugar, 2 Tablespoons granulated sugar, 1 ½ teaspoon ground cinnamon, ¼ teaspoon salt
• Pour the melted butter into the mixture and, using a fork, stir until crumbles form.  Evenly sprinkle streusel over batter.
  6 Tablespoons unsalted butter
• 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.

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.
  
  ½ cup powdered sugar, 2-3 teaspoons milk, ¼ teaspoon vanilla extract
• Drizzle over cooled cake. Allow glaze to harden, slice, and enjoy!