Begin by preheating the oven to 350 degrees Fahrenheit. Prepare a 9 x 13-inch baking dish by buttering it thoroughly, or use two 8 x 8-inch baking dishes if preferred. Set the dish aside.
In a microwave-safe mixing bowl, place the stick of unsalted butter. Heat it in the microwave until fully melted.
Next, add 1 cup of the granulated sugar, flour, baking powder, and kosher salt to the melted butter. Mix these dry ingredients together. Then, incorporate the whole milk and pure vanilla extract, beating the mixture until fully combined. Pour this batter into the prepared baking dish.
Finally, evenly distribute the sliced peaches over the batter, ensuring to place them throughout the mixture while leaving the juice in the can. Bake as directed and enjoy your delightful creation.