Directions
Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Do not wait for the oven to finish preheating before you get started — you will be melting the butter in the dish while the oven comes up to temperature, and timing it correctly is part of what makes this method work so cleanly.
Place the stick of butter, whole and still cold, directly into a 9-by-13-inch casserole dish. Slide the dish into the oven as it preheats. Watch it carefully and remove the dish as soon as the butter is completely melted and just beginning to foam at the edges. You do not want the butter to brown at this stage. Set the dish aside on a flat, heat-safe surface.
While the butter is melting, make your batter. In a large bowl, whisk together the flour, three-quarters of a cup of the sugar, and the whole milk until the batter is completely smooth and free of lumps. This is a thin, pourable batter — that is exactly what you want. Do not add any leavening agents or anything else. The simplicity of the batter is what creates that distinctive cobbler texture: dense and slightly doughy in the center where it meets the fruit juice, and beautifully crisp and golden at the edges where it meets the buttered sides of the dish.
Pour the batter gently and evenly over the melted butter in the casserole dish. Do not stir. Do not mix the batter into the butter. Simply pour it over the top and let it spread naturally. It will not cover the butter completely at first and that is completely fine.
Now scatter the rock-hard frozen peach slices evenly over the top of the batter. Again, do not stir. Do not press them down. Simply distribute them as evenly as you can across the surface of the batter and let them rest there. They will sink naturally as they bake and release their juice, and the batter will rise up around them in the way that makes cobbler cobbler.
Sprinkle the remaining quarter cup of sugar evenly over the top of the frozen peaches. This final layer of sugar is what helps create that beautifully golden, very slightly crystallized surface on top of the finished cobbler. Do not skip it and do not stir it in.
Transfer the casserole dish carefully to the preheated oven. Bake for 55 to 70 minutes, until the top of the cobbler is deeply golden brown, the edges are crisp and pulling slightly away from the sides of the dish, and the peach filling is bubbling vigorously around the edges and through any gaps in the topping. The longer baking time compared to a fresh-fruit cobbler is necessary to account for the fact that your peaches started rock-hard and frozen — they need the full time to thaw, soften, and release all their juice into the dish.
