½ cup (1 stick) plus 4 tablespoons unsalted butter, at room temperature, divided
¾ cup plus ½ teaspoon granulated sugar, divided
½ teaspoon vanilla extract
3 egg whites, divided
½ cup 2 percent milk, at room temperature
¼ cup dark brown sugar
Five ½-inch-thick cored and halved pineapple rings, from about 1 small pineapple
Directions
Preheat the oven to 325˚ and position a rack in the center.
In a small mixing bowl, sift together the flour, baking powder and salt. In the bowl of a stand mixer fitted with the paddle attachment, beat ½ cup of the butter, ¾ cup of the sugar and the vanilla at medium speed until fluffy, about 4 minutes. Add 1 of the egg whites and beat until combined. Mix in the flour mixture in three additions, alternating with the milk, until combined.
In a small bowl, whisk the 2 remaining egg whites with the remaining ½ teaspoon granulated sugar until medium peaks form, about 3 minutes. Gently fold the egg whites into the batter.
In an 8-inch cast-iron skillet, melt the remaining 4 tablespoons of butter with the brown sugar over medium heat, whisking occasionally, until the mixture begins to caramelize, about 5 minutes. Remove the skillet from the heat and arrange the pineapple in the caramel. Pour the batter over the top and smooth with a spatula.
Place the skillet in the oven and bake the cake until golden and a tester stick inserted in the middle comes out clean, about 35 minutes. Let the cake cool in the skillet for 15 minutes, then run a knife around the edge of the cake and carefully invert it onto a plate. Replace any pineapple pieces that stick to the skillet. Serve warm or at room temperature.