Slice butter into 8 segments and distribute evenly in a 9 x 13 baking dish.
Set oven temperature to 350°F and place dish in oven. The butter will melt as the oven preheats.
Mix peach slices and sugar in a saucepan and cook over medium heat for about 3-5 minutes, until sugar is dissolved and juices begin to emerge.
Remove from heat and stir in remaining filling ingredients. Set aside
In a large bowl, mix together the dry batter ingredients: flour, sugar, baking powder, and salt.
Stir in milk, just until combined. Do not overmix.
Remove the baking pan from the oven. Spoon the batter into the pan, over the melted butter. Smooth it into an even layer.
Spoon the peach filling mixture, including the juice, over the batter. Sprinkle on additional cinnamon, if desired.
Bake at 350°F for about 40 minutes.
Serve warm or cool. Add a scoop of ice cream or whipped cream just before serving, if desired.