Heat oven to 450°F and position the rack in the middle of the oven.
Pour melted butter into the bottom of the prepared pan, tilting it to evenly coat the bottom or simply melt butter in the pan in the oven for 2-3 minutes until bubbly.
In a large mixing bowl, whisk together the self-rising flour, granulated sugar, and salt.
Add the milk and stir until just combined—don’t overmix. Gently fold in the diced peaches and peach preserves (if using).
Pour the batter over the melted butter in the pan. Do not stir.
Use a butter knife or spatula to lightly score the top of the batter into 9 squares—this helps the biscuits bake more evenly and gives you clear serving portions.
In a small bowl, mix the brown sugar and cinnamon, then sprinkle evenly over the top of the batter.
Bake on the middle rack for 25–30 minutes, or until the biscuits are golden brown and fully cooked through. A toothpick inserted in the center should come out clean.
While the biscuits bake, make the glaze (if using): In a small bowl, whisk together powdered sugar, vanilla, milk, and peach preserves (if using) until smooth. Start with 4 tablespoons of milk and add more if needed to reach a pourable consistency.
Cool the biscuits for 10–15 minutes after baking. Then drizzle the glaze generously over the top, slice along the scored lines, and serve warm.