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 1/2 cups all-purpose flour, 1 cup granulated sugar, baking powder, baking soda, and salt until evenly distributed.

3. In a medium bowl, whisk together the melted butter, sourdough starter, milk, egg, and vanilla extract until well combined.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir with a spatula or wooden spoon until just combined. Do not overmix. Gently fold in the sliced peaches until evenly distributed.

5. In a separate small bowl, combine the 1/2 cup all-purpose flour, 1/4 cup granulated sugar, 1/4 cup packed light brown sugar, and ground cinnamon. Add the cold butter pieces and use your fingertips or a pastry blender to cut the butter into the flour mixture until it resembles coarse crumbs.

6. Pour the cake batter into your prepared baking pan and spread it evenly. Sprinkle the streusel topping generously over the entire surface of the batter. Place the pan in the preheated oven and bake for 35-40 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean.

7. Once baked, remove the cake from the oven and let it cool in the pan on a wire rack for at least 15-20 minutes before slicing and serving.