Whisk the flour, sugar, baking powder, baking soda, and salt together in a large bowl. Add the butter. Using a pastry cutter or two forks, cut the butter into the mixture until it resembles coarse meal (pea-sized bits). A pastry cutter makes this step very easy and quick!
While slowly stirring, slowly drizzle in the buttermilk. Mix until evenly combined.
Take handfuls of dough and gently flatten them out. Place dough all over the top of the peach filling. It doesn’t need to be perfect or look pretty– some peach filling can be showing.
Brush the top of the biscuit dough with egg wash, then sprinkle with cinnamon sugar if using.
Bake for 40-45 minutes or until lightly browned and biscuit topping is cooked through. (Stick a toothpick into the biscuit topping, if it comes out clean– it is cooked through and the cobbler is done.) Let it cool for 5 minutes before serving. Serve warm with vanilla ice cream or whipped cream.