In a large mixing bowl or stand mixer, combine the dry ingredients—flour, sugar, and salt. Add the milk and 1/4 cup of starter, stirring until just combined. Cover with plastic wrap or beeswax wrap and let sit at room temperature for about 8 hours to allow the fermentation process to take place. If needed, refrigerate for up to 48 hours.
Preheat your oven to 350°F. Add butter (sliced into chunks) to a 9x13 baking dish or large cast iron skillet. Let it melt in the oven while it preheats. This creates the perfect base for the cobbler dough
While the butter melts, prepare your fruit. In a separate bowl or saucepan, combine the fresh peaches, sugar, and salt. Simmer briefly on medium heat until the sugar dissolves and the peaches release juices.
Mix the baking powder into your fermented batter. This optional step helps lift the dough into a soft, airy finish, but if you allowed your batter to long ferment, you can skip the baking powder if desired.
Remove the pan with melted butter from the oven. Pour the batter directly over the butter—do not stir, just spread into an even layer. Spoon the peach mixture (with all that syrupy juice!) over the batter. Sprinkle cinnamon generously over the top of the fruit.
Bake for about 40 minutes, or until the cobbler is golden brown and the fruit is bubbling at the edges. Let the cobbler cool slightly before serving.
Enjoy warm with a scoop of ice cream, if desired. Store leftovers in an airtight container in the fridge for up to 4 days. It's just as good the next day—if not better.