Step 1: Preheat your oven to 425°F (220°C). Pour the melted butter into a 9x13 inch baking dish, tilting to coat the bottom evenly.
Step 2: In a large bowl, whisk together the self-rising flour, granulated sugar, baking powder, and salt. If using, whisk in the cinnamon.
Step 3: Pour the milk into the dry ingredients and stir gently with a spatula or wooden spoon until just combined. The dough will be shaggy and sticky – do not overmix.
Step 4: Carefully pour the biscuit dough directly over the melted butter in the baking dish. Do not stir or mix the dough into the butter.
Step 5: Spread the dough evenly across the dish using the back of a spoon or spatula. Top evenly with the diced peaches, pressing them lightly into the dough.
Step 6: Sprinkle a little extra sugar over the top of the peaches and dough for a sweet crust.
Step 7: Bake for 25-30 minutes, or until the biscuits are golden brown on top and cooked through. The butter will bubble up around the edges and create a delicious "swim" sauce.
Step 8: Remove from oven and let cool slightly in the pan for 5-10 minutes. The biscuits are best served warm, straight from the pan.