- 3 tablespoons unsalted butter
- 1/2 cup packed light brown sugar
- 1 pound Granny Smith apples, peeled, cored, and cut into thin wedges
- 1 cup all-purpose flour
- 1/4 cup granulated sugar
- 1 teaspoon baking powder
- 1/2 teaspoon baking soda
- 1/2 teaspoon salt
- 1/2 teaspoon cinnamon
- 5 tablespoons cold unsalted butter, cut into pieces
- 1/2 cup well-shaken buttermilk
- Accompaniment: creme fraiche or sour cream (optional)
- Preheat oven to 425 degrees F.
- Make topping: Heat butter in an ovenproof 10-inch heavy skillet (preferably well-seasoned cast-iron) over moderate heat until foam subsides. Stir in brown sugar and remove from heat. Spread mixture evenly in skillet and arrange apples, overlapping, in 1 layer.
- Make cake: Blend flour, sugar, baking powder and soda, salt, and cinnamon in a food processor. Add butter and pulse until mixture resembles coarse meal. Transfer to a bowl and add buttermilk, stirring just until mixture is moistened.
- Drop batter on top of apples and gently spread, leaving a 1-inch border around edge of skillet. (Cake needs room to expand.)
- Bake cake in middle of oven until golden brown and firm to the touch, 25 to 30 minutes. Cool cake in skillet on a rack 3 minutes, then invert onto a platter. Replace any apples that stick to skillet on cake.
- Serve warm.