1Preheat oven to 350 degrees F (175 degrees C). Grease and flour 2 (8 inch) round pans.
2In a large bowl, cream together 1/2 cup butter and 1 1/2 cups sugar until light and fluffy. Beat in the egg yolks one at a time. Dissolve soda in buttermilk; add to creamed mixture alternately with flour, beginning and ending with flour. Stir in the mashed bananas and pecans.
3In a large glass or metal mixing bowl, beat egg whites until stiff peaks form. Fold 1/3 of the whites into the batter, then quickly fold in remaining whites until no streaks remain. Pour batter into prepared pans.
4Bake in the preheated oven for 30 to 35 minutes, or until a toothpick inserted into the center of the cake comes out clean. Let cool in pans for 10 minutes, then turn out onto a wire rack and cool completely. Spread Caramel Frosting between layers and on top and sides of cooled cake.
5To make the Caramel Frosting: In a heavy saucepan, combine 1 1/4 cup sugar, 3/4 cup brown sugar, evaporated milk and 1/2 cup butter. Cook over medium heat, stirring frequently, until mixture reaches softball stage (240 degrees F; 116 degrees C). Remove from heat, and beat until spreading consistency; add additional milk if necessary. Spread immediately on cooled cake.