Preheat oven to 350 degrees.
Cut butter into pieces and put into 9x13 dish to melt in the oven (about 2 minutes).
Mix flour, sugar, baking powder, and salt together in a mixing bowl.
Add milk and peach syrup and mix well. Batter will be thin but not runny.
Pour batter into dish over melted butter. Do not stir.
Add peach slices on top of batter and sprinkle with cinnamon.
Bake approximately 1 hour until inserted toothpick comes out clean.