Heat grill to 350 degrees F.
Place a 10-inch cast iron skillet in the center of the grill and heat thoroughly.
Meanwhile, make cake batter: Sift the cake flour, baking powder, and salt together; set aside. With a mixer, cream together the butter, sugar, and egg yolks. With the mixer on low speed, add the dry ingredients to creamed mixture, alternating with the milk. Add vanilla extract. In a separate mixing bowl, beat the egg whites until stiff peaks form and then fold them into the batter. Set batter aside.
Place the 1/2 stick butter and the brown sugar in the skillet and let the mixture melt and begin to caramelize. When lightly caramelized, place pineapples and enough cherries to cover on top, in a decorative pattern. Then pour the cake batter over, filling to within 1/2-inch of the top of the skillet. Transfer the skillet to a large foil pan and cover with foil; be careful handling the skillet as the handle will also be very hot. Return to the grill and pierce the foil in the corners to allow steam to escape during the cooking process.
Grill cake at 350 degrees F until cake is golden brown and springs back when lightly touched with your finger, about 45 minutes. Remove from the grill and run a knife around the edge of the skillet to free the cake; again, be careful handling the skillet as the handle will also be very hot. While cake is still hot, place a serving plate on top of the skillet and carefully turn the cake over onto the plate. Serve warm or cold.