Beat butter and sugar in a large bowl with an electric mixer until light and fluffy
Add eggs one at a time, beating well after each addition
Add vanilla and rum
Incorporate half of the flour, baking powder, and salt and mix on low just until blended
Add pineapple juice and beat again until blended
Add remaining flour, again just until blended
Batter may appear slightly curdled
Spoon batter over pineapple in the cast iron pan and spread evenly until pineapple slices are covered
Bake at Fahrenheit 350° for 35 to 45 minutes
Remove from oven and let sit for 5 minutes
Carefully invert cake onto a plate
Serve cake warm or at room temperature