1. 1. Prepare Your Baking Pan and Oven: Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan, or line it with parchment paper, leaving an overhang on two sides for easy removal. This ensures your cake won’t stick and comes out cleanly.

2. 2. Mix the Dry Cake Ingredients: In a large mixing bowl, whisk together the 1 cup all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and everything is evenly distributed.

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

4. 4. Combine Wet and Dry for the Cake Batter: Pour the wet ingredients into the dry ingredients. Stir with a spatula or wooden spoon until just combined. Be careful not to overmix; a few small lumps are okay. Overmixing can lead to a tough cake. Pour the batter evenly into your prepared baking pan and spread it out.

5. 5. Prepare the Crumb Topping: In a medium bowl, combine the 1/2 cup all-purpose flour, 1/4 cup granulated sugar, brown sugar, and cinnamon. Add the cold, cubed butter. Using your fingertips or a pastry blender, work the butter into the dry ingredients until coarse crumbs form. You want a mixture that resembles wet sand with some larger pea-sized pieces. Sprinkle this crumb topping evenly over the cake batter in the pan.

6. 6. Bake the Crumb Cake: Place the baking pan in the preheated oven. Bake for 25-30 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out with moist crumbs, but not wet batter. The crumb topping should be golden brown.

7. 7. Cool and Glaze (Optional): Remove the cake from the oven and let it cool in the pan for about 10 minutes. While it cools, whisk together the powdered sugar, 1-2 tablespoons of milk or water, and 1/4 teaspoon vanilla extract for the optional glaze until smooth. If the glaze is too thick, add a tiny bit more liquid; if too thin, add a little more powdered sugar. Drizzle the glaze over the warm cake before slicing and serving.