1. 1. Prepare Your Baking Dish and Oven: Start by preheating your oven to 350 F (175 C). Grease and flour an 8×8-inch square baking dish or a 9-inch round cake pan.

2. 2. Whisk Dry Ingredients for the Cake: In a large mixing bowl, combine the 1 1/2 cups all-purpose flour, 1 cup granulated sugar, baking powder, baking soda, and 1/4 teaspoon salt. Use a whisk to thoroughly combine these ingredients.

3. 3. Combine Wet Ingredients: In a separate medium bowl, whisk together the 1/2 cup melted butter, egg, vanilla extract, sourdough discard, and 1/2 cup buttermilk until smooth.

4. 4. Combine Wet and Dry Mixtures: Pour the wet ingredient mixture into the dry ingredients. Stir with a spatula or wooden spoon until just combined. Do not overmix.

5. 5. Prepare the Crumb Topping: In a small bowl, combine the 1/2 cup all-purpose flour, 1/4 cup granulated sugar, brown sugar, and 1/4 teaspoon ground cinnamon for the topping. Add the 1/4 cup cold, cubed butter. Using your fingertips or a pastry blender, work the butter into the dry ingredients until the mixture resembles coarse crumbs.

6. 6. Assemble and Bake: Pour the cake batter into your prepared baking dish and spread it evenly. Generously sprinkle the crumb topping over the entire surface of the batter. Place the dish in the preheated oven and bake for 30-35 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean.

7. 7. Cool Before Serving: Once baked, remove the coffee cake from the oven and place it on a wire rack to cool for at least 15-20 minutes before slicing and serving.