Heat the oven to 350°F (180°C).
In a 10-inch (26 cm) cast iron or stainless steel skillet, melt the butter with the sugar and salt over medium heat. Cook 4 to 5 minutes without stirring, until the caramel turns amber.
Take the skillet off the heat and arrange the apple wedges tightly in a circle, rounded side down. Return to medium heat and cook 5 minutes, until the apples start to soften and release juice.
In a bowl, whisk the eggs with the sugar, oil, milk and vanilla. Add the flour, baking powder and cinnamon and mix just until no dry flour remains.
Pour the batter over the apples and gently spread it to the edges.
Bake 35 to 40 minutes, until a skewer inserted in the center comes out dry.
Let it rest 10 minutes. Run a knife around the edge, set a serving plate upside down over the skillet and flip in one move, wearing oven mitts.
