Grease an 8x8 inch baking dish with cooking spray. Preheat oven to 400°F.
Melt 4 tablespoons of the butter in a small dish in the microwave. Place the remaining 2 tablespoons of butter in the refrigerator.
Place ⅔ cup sugar, cornstarch, and water in a saucepan over medium heat. Bring to a boil. Boil for 1 minute, then remove from the heat. Stir in the peaches.
Pour the mixture into the prepared baking dish. Sprinkle the cinnamon over the top.
Cut the remaining 2 tablespoons of butter into 8 pieces, and scatter the butter over the peaches.
In a large bowl, stir together the Bisquick, melted butter, milk, and sugar until a dough forms. Dollop spoonfuls of the dough onto the peaches.
Bake in the preheated oven for 20–25 minutes, or until the dough is lightly browned on top.
Serve with ice cream or whipped cream.