Set a 10" cast iron skillet over medium low heat, and add the cubed butter.
Once melted, add the brown sugar- whisking until it's dissolved.
Remove the skillet from heat, then stir in the orange juice until combined. Add the blueberries evenly out into the dish.
In a separate mixing bowl, add the flour, sugar, and baking powder. Whisk together to evenly combine.
In another mixing bowl, whisk together the egg, milk, melted butter, and vanilla extract- to evenly combine.
Pour the egg mixture into the bowl with the flour mixture, and stir just until evenly combined. Do not over mix.
Pour the batter out over the blueberries and sauce in the skillet, spreading gently with a spatula (if needed) for an even layer.
Bake the cake at 400° for 20-25 minutes, or until a toothpick inserted in the center comes out clean.
Remove the skillet from the oven, and allow it to cool for 10-15 minutes before turning it out onto a waiting serving platter.