Assembly

• Preheat the oven to 350 degrees Fahrenheit and grease a 9x13-inch baking dish. Ensure your oven is fully up to temperature before you begin mixing the wet and dry ingredients, as the leavening reaction starts immediately upon contact.
• 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. The crumble should feel like wet sand and hold its shape when pressed together.
• In a large mixing bowl, cream the oil, cane sugar, and brown sugar until smooth. Because we are using oil, you won't get the same aeration as you would with butter, but you want to ensure the sugars are evenly distributed throughout the lipid base.
• Add the eggs, vanilla, sourdough starter, and milk. Mix well. You will notice the mixture becomes quite thin and pale. The sourdough starter may require a bit of whisking to fully incorporate into the milk and eggs.
• 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. Stop the very second you no longer see streaks of white flour (do not overmix).
• 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. Use a gentle hand when spreading the second layer of batter to avoid disturbing the middle layer of crumble.
• 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.