• Preheat the oven to 350° F and grease or line a 9×13-inch cake pan with parchment paper.

Make the Crumble Topping

• In a medium mixing bowl, add the sugar, flour, salt, and cinnamon. Stir to combine.
  1 cup granulated sugar, 1 cup all-purpose flour, 1/4 teaspoon salt, 1 tablespoon cinnamon
• Add the melted butter and stir until combined and the texture is crumbly. Set aside.
  6 tablespoons unsalted butter

Make the Filling

• In a small bowl, combine the brown sugar and cinnamon. Set aside.
  1/2 cup dark brown sugar, 1 1/2 tablespoons cinnamon

Make the Cake

• In a medium-sized mixing bowl, whisk together the sour cream and milk. It's okay if there are some lumps. Set aside.
  3/4 cup sour cream, 1 1/4 cup milk
• In the bowl of a stand mixer, add the butter, salt, granulated sugar, brown sugar, baking powder, and vanilla. Use the paddle attachment on medium speed to mix the ingredients until they are combined and smooth.
  12 tablespoons unsalted butter, 1 teaspoon salt, 1 1/2 cups granulated sugar, 1/3 cup dark brown sugar, 2 1/2 teaspoons baking powder, 2 teaspoons vanilla extract
• Add one egg at a time, and mix on medium speed after each egg. Add the sourdough starter with the last egg. This will help the starter incorporate more easily.
  3 large eggs, 1/3 cup sourdough starter
• Add 1/3 of the flour to the butter mixture. Mix on low speed just until combined.
  3 1/4 cup all-purpose flour
• Add 1/3 of the sour cream-milk mixture. Mix on low speed just until combined. Alternate adding the remaining flour and milk, 1/3 at a time, mixing between each addition. Do not overmix.
• Pour half of the cake batter into the prepared cake pan. Use a silicone spatula or spoon to spread evenly and to the edges of the pan.
• Sprinkle the filling evenly on the cake batter in the pan.
• Spoon large dollops of the remaining batter on top. Try to add the dollops evenly across the pan.
• Use a table knife to gently swirl and spread the batter to the edges of the pan in a zig-zag pattern, like you would with a marble cake.
• Sprinkle the crumble topping over the cake.
• Bake for 60 minutes or until a toothpick inserted in the center comes out clean.
• Allow to cool for 30 minutes or so before serving.