Mix flour, salt, and baking soda in a large bowl.
Combine sourdough starter, eggs, sugar, sour cream, Greek yogurt, or skyr, milk, melted butter, and vanilla, in a small bowl until smooth.
Add wet ingredients to dry ingredients until smooth. The batter should have a typical thick cake batter consistency, pourable but somewhat holding its shape.
Cover bowl and let batter ferment at room temperature or in a fairly warm place for 4-5 hours.
After fermenting for 4 or 5 hours, fold optional add-ins into batter, if using.
Prepare cake pan by greasing the bottom and sides with butter.
Pour half of the batter into the prepared pan and carefully spread into an even layer.
Evenly distribute half of the crumble filling/topping over the layer of batter.
Pour the rest of the batter, scraping the batter from the bowl with a rubber spatula as needed, and spread evenly.
Distribute the rest of the crumble filling/topping over top layer of batter.
Preheat oven to 350 degreed Fahrenheit.
Place unbaked cake in the center rack of the oven and bake for 40 to 45 minutes, or until the top is slightly golden browm and a toothpick comes out clean when inserted into the center.
Remove cake from oven and let rest for 15 minutes before cutting into.
Enjoy!