To peel the plantains, cut off ends and discard. With a paring knife, make 3 shallow slits lengthwise along the seams of the skin and peel away. Split the plantains lengthwise.
Heat 4 tablespoons butter in a large skillet and place over medium low heat (plantains have a high sugar content and will burn if the heat is too high.) Fry the plantains in a single layer, until golden on the bottom then turn over with a spatula. Add the cinnamon, vanilla, and sugar and let cook a few more seconds, just to begin caramelizing the sugar.
Just before serving, melt remaining tablespoon of butter to finish the sauce. Serve immediately.