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 for easy removal.

2. In a medium bowl, combine 1/2 cup all-purpose flour, 1/2 cup granulated sugar, 1/2 cup packed light brown sugar, and 1/2 teaspoon ground cinnamon. Pour in the 1/2 cup melted butter and mix with a fork until large, moist crumbs form. Set aside.

3. In a large bowl, whisk together the remaining 1 cup all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Make sure there are no lumps.

4. In a separate medium bowl, whisk the large egg, 1/2 cup sourdough discard, 1/2 cup milk, and 1 teaspoon vanilla extract until well combined and smooth.

5. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few small lumps are perfectly fine. Overmixing can lead to a tough cake.

6. Pour the cake batter evenly into your prepared 8×8 inch baking pan. Sprinkle the crumb topping generously and evenly over the batter. Place the pan in the preheated oven and bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The crumb topping should be golden brown.

7. Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving. This allows the cake to set properly and prevents it from crumbling too much when cut.