Preheat the oven to 350 degrees Fahrenheit and grease a 9x13-inch baking dish.

In a medium-sized mixing bowl, stir the cinnamon crumble ingredients (melted and cooled butter, brown sugar, cinnamon, and flour) until completely combined. Set aside.

In a large mixing bowl, cream the oil, cane sugar, and brown sugar until smooth.

Add the eggs, vanilla, sourdough starter, and milk. Mix well.

In a separate bowl, whisk the salt, baking soda, and flour until combined. Add the dry ingredients to the wet ingredients and fold with a spatula until just combined. Don't overmix the batter.

Pour half of the coffee cake batter into the prepared baking dish. Sprinkle half of the crumble mixture evenly on top.

Pour the remaining cake batter over the crumble, then sprinkle the rest of the crumble mixture over the top of the batter.

Bake in the preheated oven for 40-50 minutes. The edges will start to turn golden, and a toothpick inserted should come out clean. Cool completely before slicing and serving.