Pre-heat the oven to 400 F.
Put the butter in your cast-iron skillet (or medium oven-proof dish) and place in the oven while it pre-heats.
Meanwhile, in a medium bowl, whisk 1 cup of the sugar with the flour and milk, until smooth.
Take the skillet/dish out of the oven and swirl the melted butter around to evenly coat the bottom of the pan/dish.
Pour the batter into the skillet/dish.
Sprinkle the blueberries evenly over the top.
Sprinkle on the remaining 1/4 cup of sugar over the top.
Bake until golden and bubbly! About 40 to 45 minutes.
Serve piping hot, topped with a big scoop of vanilla ice cream.