• Preheat oven to 350℉. Spray the bottom and sides of a springform pan with baking spray.
  baking spray

Prepare Crumb Topping

• In a medium bowl, add flour, brown sugar, salt, and cubed butter. Use a pastry blender or a fork to press them together until the mixture resembles coarse crumbs.
  ⅔ cup light brown sugar , 1 cup all-purpose flour, ¼ teaspoon salt, ½ cup unsalted butter
• Set aside in the fridge to chill.

Prepare Cake Layer

• In a large mixing bowl or the bowl of a stand mixer, mix together flour, white and brown sugar, baking soda, baking powder, and salt.
  2 cup all-purpose flour, 1 ½ teaspoon baking powder, ¼ teaspoon baking soda, ½ teaspoon salt, ⅔ cup light brown sugar, ¼ cup granulated sugar
• Add in butter and oil, and mix well until dry ingredients are well coated.
  ½ cup unsalted butter, ¼ cup vegetable oil
• Mix in eggs and vanilla, followed by sour cream until batter is combined.
  2 large eggs, 2 teaspoons pure vanilla extract, ¾ cup sour cream
• Spread batter evenly into prepared springform pan. Top evenly with blackberries, followed by crumble topping.
  1 ½ cups blackberries
• Bake in preheated oven for 50-55 minutes, or until a toothpick entered into the center of the cake comes out clean.
• Let cake cool in the pan 15 minutes, then remove to a wire rack to cool completely.
• To make vanilla icing, whisk together powdered sugar, vanilla, and milk. Add a little extra milk if needed for desired consistency. Drizzle icing over cooled cake.
  1 cup powdered sugar, ½ teaspoon vanilla extract, 1 tablespoon milk