1. 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.

2. In a large bowl, whisk together the 1 1/2 cups all-purpose flour, 1/2 cup granulated sugar, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Make sure there are no lumps and everything is well combined.

3. In a separate medium bowl, whisk together the 1/2 cup melted unsalted butter, 1/2 cup sourdough discard, 1/2 cup milk, 1 large egg, and 1 teaspoon vanilla extract until smooth and fully incorporated.

4. 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 fine. Overmixing can lead to a tough cake.

5. In a small bowl, combine the 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 cup packed light brown sugar, and 1/4 teaspoon ground cinnamon. Add the 1/4 cup cold, cubed unsalted butter. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs.

6. Pour the cake batter into your prepared baking pan and spread it evenly. Sprinkle the crumb topping generously over the batter. Place the pan 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. The topping should be golden brown.

7. Remove the coffee cake from the oven and let it cool in the pan on a wire rack for at least 10-15 minutes. While it cools, prepare the glaze by whisking together the 1/2 cup powdered sugar, 1-2 tablespoons milk or water, and 1/4 teaspoon vanilla extract until smooth. Drizzle the glaze over the slightly warm or fully cooled cake before slicing and serving.