1. Preheat the oven to 350°F (175°C) and grease or line an 8-inch round or 9×9-inch square baking pan.

2. In a bowl whisk together flour, baking powder, baking soda, salt, and ground cinnamon until combined.

3. In another bowl whisk together sourdough starter discard, melted butter, eggs, granulated sugar, milk, and vanilla extract.

4. Pour the dry ingredients into the wet ingredients and fold gently with a spatula until just combined.

5. In a small bowl mix brown sugar, a tablespoon of flour, cinnamon, and a few tablespoons of melted butter to form the streusel crumb.

6. Spread half of the batter into the prepared pan and sprinkle half of the streusel evenly over the batter.

7. Top with the remaining batter and scatter the rest of the streusel mixture over the top.

8. Bake for 35 minutes or until the cake is golden and a toothpick inserted in the center comes out clean.

9. Allow the cake to cool in the pan for 10–15 minutes before slicing and serving. Enjoy warm or at room temperature.