1. Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan.

2. In a large mixing bowl, whisk together the all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt until evenly distributed.

3. In a separate medium bowl, whisk together the sourdough discard, milk, 1/4 cup melted butter, egg, and vanilla extract until well combined and smooth.

4. Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Do not overmix.

5. Pour the batter evenly into your prepared baking pan. In a small bowl, whisk together the 1/4 cup granulated sugar and ground cinnamon for the topping. Drizzle the 2 tablespoons of melted butter over the cake batter, then sprinkle the cinnamon sugar mixture evenly over the top.

6. Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out clean or with only a few moist crumbs attached. The top should be golden brown.

7. Remove the cake from the oven and let it cool in the pan on a wire rack for about 10-15 minutes before slicing and serving.