Step 2: Place the butter in a large (10- to 12-inch) cast-iron skillet (may substitute a large ovenproof baking dish) and place in the oven so the butter melts; this should take 5 to 7 minutes.
Step 3: Meanwhile, place the fruit in a mixing bowl. Use a potato masher to mash it just enough so that the pieces release some of their juices. If the fruit is tart, sprinkle a little sugar on top.
Step 4: Whisk together the flour, baking powder and salt in a medium bowl. Add the cup of sugar, milk and vanilla extract, stirring to form a smooth batter.
Step 5: Remove the skillet from the oven. Carefully pour the melted butter from the skillet into the batter, stirring to combine, then pour the batter all at once into the skillet (it should begin to puff and set around the edges). Add the fruit and its juices to the center of the batter.
Step 6: Bake for 35 to 40 minutes, until the top is golden-brown and a toothpick inserted into the crumb comes out mostly clean. Serve warm, straight from the skillet.