1. Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8-inch baking pan, or line it with parchment paper, leaving an overhang on the sides.

2. In a large mixing bowl, whisk together the unfed sourdough discard, melted butter, granulated sugar, egg, and vanilla extract until smooth and well combined.

3. In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, 1/2 teaspoon ground cinnamon, and salt.

4. Gradually add the dry ingredient mixture to the wet ingredients, mixing until just combined. Do not overmix.

5. Gently fold in the finely diced apple, distributing the pieces evenly throughout the batter.

6. Pour the batter into the prepared 8×8-inch baking pan and spread it evenly with a spatula. In a small bowl, combine the 2 tablespoons of brown sugar and 1/2 teaspoon of ground cinnamon for the topping. Sprinkle this mixture evenly over the top of the batter.

7. Bake for 20-25 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The top should be golden brown and the edges slightly firm.

8. Remove the pan from the oven and let the bars cool completely in the pan on a wire rack before slicing into squares.