1. Preheat your oven to 350 F (175 C). Lightly grease and flour an 8×8 inch square baking pan or a 9-inch round cake pan.

2. In a large mixing bowl, whisk together the 1 cup all-purpose flour, 1/2 cup granulated sugar, 1/4 cup packed light brown sugar, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Make sure there are no lumps.

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

4. Pour the wet ingredient mixture into the dry ingredients. Stir with a spatula or wooden spoon until just combined. Do not overmix. Pour the batter evenly into your prepared baking pan.

5. In a small bowl, combine the 1/2 cup all-purpose flour, 1/4 cup packed light brown sugar, 1/4 cup granulated sugar, and 1/4 teaspoon ground cinnamon. Pour the 1/4 cup melted butter over the dry crumb ingredients. Use a fork or your fingertips to mix until coarse crumbs form. Sprinkle this mixture evenly over the cake batter in the pan.

6. 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 top should be golden brown and the crumbs lightly toasted.

7. While the cake cools slightly, whisk together the 1/2 cup powdered sugar and 1-2 tablespoons milk in a small bowl until smooth. Start with 1 tablespoon of milk and add more gradually until you reach your desired drizzling consistency.

8. Let the coffee cake cool in the pan on a wire rack for at least 10-15 minutes before drizzling with the optional glaze. Once glazed, let the glaze set for a few minutes before slicing and serving warm or at room temperature.