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

2. In a large bowl, combine the 1 1/2 cups all-purpose flour, 1 cup granulated sugar, baking powder, baking soda, and salt. Whisk them together thoroughly until well combined.

3. In a separate medium bowl, whisk together the melted butter, egg, vanilla extract, sourdough discard, and milk until smooth.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula or wooden spoon until just combined. Do not overmix.

5. In a small bowl, combine the 1/2 cup flour, 1/4 cup granulated sugar, brown sugar, and cinnamon for the streusel. Add the cold, diced 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 streusel 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 top should be golden brown.

7. Remove the cake from the oven and let it cool in the pan on a wire rack for at least 15 minutes. If making the optional glaze, whisk together the powdered sugar and 1-2 tablespoons milk/cream until smooth. Drizzle the glaze over the slightly warm or fully cooled cake just before serving.