Preheat oven to 375°F (190°C)
Grease a 9×9-inch baking dish
In a bowl, mix 1 can (15 oz) sliced peaches (drained) or 4 cups fresh/frozen peaches with 1/3 to 1/2 cup sugar and 1 tbsp cornstarch (optional) and 1 tsp cinnamon
Pour peach mixture into the baking dish
In a bowl, mix 1 cup flour, 1 cup sugar, 2 tsp baking powder, and 1/2 tsp salt
Cut in 1/2 cup cold butter until crumbly (or use 1/2 cup melted butter, if preferred)
Stir in 1 cup milk until just combined
Spoon batter over the peaches
Bake 35 to 45 minutes, until golden and bubbly
Let cool 10 to 15 minutes before serving
Serve with vanilla ice cream or whipped cream (optional)