How to Make It
Preheat oven to 350°F. Beat butter with an electric mixer on medium speed until smooth, about 30 seconds. Gradually add brown sugar, beating until light and fluffy, about 2 minutes. Spread mixture evenly in a 9-inch cast-iron skillet. Arrange pineapple slices and cherries evenly over mixture, cutting pineapple slices as needed to fit skillet.
Beat egg yolks on high speed until thick and lemon-colored, about 2 minutes. Gradually add granulated sugar, beating well to combine. Stir together flour and salt in a separate bowl; add to yolk mixture alternately with water, stirring well after each addition. Stir in vanilla.
Beat egg whites on medium-high speed until stiff peaks form, about 2 minutes; fold into batter.
Spoon batter evenly over pineapple and cherries in skillet. Bake in preheated oven until a wooden pick inserted in center comes out clean, 45 to 50 minutes. Cool cake in skillet 30 minutes. Invert cake onto a serving plate.