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

2. In a large bowl whisk melted butter, brown sugar and vanilla until smooth. Beat in egg until combined.

3. Stir in sourdough discard until evenly blended.

4. In a separate bowl whisk flour, baking powder, baking soda, salt and cinnamon. Fold the dry mixture into the wet mixture in two additions, mixing gently until just combined.

5. In a small bowl combine brown sugar, flour and cinnamon for the crumble. Cut in cold butter with a fork until the mixture resembles coarse crumbs.

6. Spread batter into prepared pan and sprinkle the crumble topping evenly over the surface.

7. Bake for 35 to 40 minutes or until a toothpick inserted in the center comes out clean and the top springs back to the touch.

8. Let the cake cool in the pan for 10 to 15 minutes before slicing and serving warm or at room temperature.