Preheat the oven to 350 degrees.

Grease a 9×13 baking dish with coconut oil or butter.

Melt butter.

In a medium mixing bowl, combine the melted butter with brown sugar, cinnamon, and flour.

In a separate bowl, mix together the cinnamon filling. Set aside.

In a stand mixer or large mixing bowl, cream together melted coconut oil, brown sugar, and regular sugar, until well combined.

Add egg, vanilla extract, sourdough starter, water, and milk to the sugar mixture; combine.

In a separate bowl, mix together the dry ingredients.

Slowly incorporate the wet ingredients to the dry ingredients. I like to turn on my stand mixer to low speed. Then, I add about a third of the dry ingredients at a time until it comes together.

Place half of the cake mixture into the baking dish. Spread the cinnamon filling over the cake mixture evenly.

Spread the remaining cake batter over the cinnamon filling, then sprinkle the crumb topping on top.

Bake for about 45 minutes or until a toothpick comes out clean (the cinnamon topping will still stick to the toothpick, but the cake should be clean) and the edge of the cake starts to turn golden.