1. 1. Prepare your baking pan and oven: Preheat your oven to 350 F (175 C). Grease and flour a 9×5-inch loaf pan, or line it with parchment paper, leaving an overhang on the long sides to easily lift the loaf out later. This ensures your cake won’t stick.

2. 2. Combine dry ingredients for the loaf: In a large mixing bowl, whisk together 1 cup of the all-purpose flour, granulated sugar, baking soda, and salt until well combined. Make sure there are no lumps of baking soda.

3. 3. Mix wet ingredients for the loaf: In a separate medium bowl, whisk together the melted and cooled butter, sourdough discard, milk, egg, and vanilla extract until smooth. The mixture should be uniform in color and consistency.

4. 4. Combine wet and dry ingredients: Pour the wet ingredient mixture into the dry ingredient mixture. Stir with a spatula or wooden spoon until just combined. Be careful not to overmix; a few small lumps are fine. Overmixing can lead to a tough loaf. Pour the batter into your prepared loaf pan and spread evenly.

5. 5. Prepare the crumb topping: In a small bowl, combine the remaining 1/2 cup all-purpose flour, brown sugar, and cold butter pieces. Use your fingertips or a pastry blender to cut the butter into the flour and sugar until the mixture resembles coarse crumbs. Some larger pea-sized pieces are desirable for a good crumb texture.

6. 6. Add the crumb topping and bake: Sprinkle the crumb topping evenly over the batter in the loaf pan. Place the loaf pan in the preheated oven and bake for 40-50 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean. If the topping starts to brown too quickly, you can loosely tent the loaf with aluminum foil.

7. 7. Cool the loaf: Once baked, remove the loaf from the oven and let it cool in the pan for about 10-15 minutes. This allows the cake to set and makes it easier to remove. Then, using the parchment paper overhang or a knife around the edges, carefully lift the loaf out of the pan and transfer it to a wire rack to cool completely before slicing and serving.