Combine butter and brown sugar in a 12-inch cast iron skillet and heat until bubbly. Add pineapple and caramelize to a deep golden brown, turning once. Remove from skillet and set aside. Pour rum and cream into a skillet and cook over low heat 5 minutes, until thickened, stirring constantly with a whisk. Remove from heat.
Butter a 10-inch round cake pan and line with parchment paper. Arrange pineapple rings to cover bottom of pan. Pour cooled sauce over pineapple and set pan aside.
Sift together flour, baking powder, soda and salt. Set aside
In a large bowl cream butter and sugar until light and fluffy. Add egg yolks, one at a time, beating after each addition. Add vanilla. Combine pineapple juice and buttermilk. Alternately add sifted dry ingredients and juice/buttermilk mixture to creamed mixture (begin and end with dry ingredients.) Whip egg whites to soft peaks and fold into batter. Bake at 350 degrees about 1 hour, or until a tester comes out clean. Invert onto a serving platter.