Step 1. Prepare the Batter.
In a large mixing bowl, combine the starter (or sour cream and Greek yogurt), flour, sugar, baking soda, salt, cinnamon, oil, and egg. Mix until the batter is smooth and well combined.

Step 2. Prepare the Topping.
In a separate bowl, combine the flour, brown sugar, butter, and cinnamon. Mix with a fork until the mixture becomes crumbly.

Step 3. Assemble the Cake.
Preheat the oven to 350°F (175°C). Grease an 8x8-inch baking pan. Pour the batter into the pan and spread it evenly. Sprinkle the crumb topping evenly over the batter.

Step 4. Bake.
Place the pan in the oven and bake for 20–30 minutes, or until a toothpick inserted into the center comes out clean.

Step 5. Cool and Serve.
Let the cake cool slightly before slicing. Enjoy warm or at room temperature with coffee or tea.