1. Preheat your oven to 350 F (175 C). Lightly grease and flour a 9×13 inch baking pan, or line it with parchment paper, leaving an overhang on the sides to easily lift the finished slice out later.

2. In a large mixing bowl, whisk together the sourdough starter discard, egg, 1/4 cup granulated sugar, melted butter, and vanilla extract until well combined and smooth.

3. In a separate medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and 1/4 teaspoon salt. Ensure there are no lumps and the dry ingredients are evenly distributed.

4. Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Start and end with the dry ingredients. Mix until just combined. Be careful not to overmix; a few small lumps are perfectly fine.

5. Gently fold in the chocolate chips until they are evenly distributed throughout the batter.

6. Pour the batter into your prepared baking pan and spread it evenly with a spatula. In a small bowl, mix together the 2 tablespoons of granulated sugar and 1 teaspoon of ground cinnamon for the topping. Sprinkle this cinnamon sugar mixture generously over the top of the batter.

7. Bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center comes out clean. The top should be golden brown, and the edges should be slightly pulled away from the sides of the pan.

8. Once baked, remove the pan from the oven and let it cool on a wire rack for at least 15-20 minutes before slicing and serving.