1. 1. Preheat and Prepare: Begin by preheating 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. This prevents sticking and ensures easy removal.

2. 2. Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until they are well combined. Make sure there are no lumps.

3. 3. Mix Wet Ingredients: In a separate medium bowl, whisk together the active sourdough starter, milk, melted unsalted butter, large egg, and vanilla extract. Whisk until the mixture is smooth and uniform.

4. 4. Combine Wet and Dry: Pour the wet ingredients into the dry ingredients. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix, as overmixing can lead to a tough cake. A few small lumps are perfectly fine.

5. 5. Fold in Peaches: Gently fold in the sliced peaches into the batter. Distribute them evenly throughout the mixture. If using frozen peaches, there’s no need to thaw them first; just add them directly to the batter.

6. 6. Transfer to Pan and Top: Pour the batter into your prepared baking pan and spread it evenly. In a small bowl, mix the brown sugar and ground cinnamon. Sprinkle this mixture evenly over the top of the batter in the pan.

7. 7. Bake Until Golden: 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 and the cake should spring back lightly when touched.

8. 8. Cool and Serve: 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. This allows the cake to set properly and makes for cleaner slices.