Place a 10, 11, or 12-inch cast iron skillet on a center rack in the oven. Preheat your oven to 350F. Once the oven is adequately heated, allow the cast iron to remain in there for another 10-15 minutes.
Whisk together flour, baking powder, and salt in a mixing bowl. Make a well in the center. Add the milk, 1/4 cup melted butter or oil, applesauce, sugar, and vanilla. Mix until you have an even batter—some small lumps are OK.
Remove the hot skillet from the oven very carefully, using oven mitts. Add the remaining tablespoon butter or oil to the pan and swirl it around to coat. Place the peaches and blackberries into the hot skillet and immediately pour the batter over them. Transfer the skillet back to the oven.
Bake the cake for 35-45 minutes, until golden brown on top. Let cool for at least an hour before cutting into wedges and serving. Enjoy!