In a large mixing bowl, combine the cake mix, baking soda and eggs. Measure the Coca-Cola, stir briskly until the foaming stops; add to the batter. Blend ingredients just until moistened, then beat at high speed of an electric mixer for 3 minutes, scraping the bowl often.
In a bowl, combine the mashed bananas with the lemon juice; add to the batter. Add the chopped nuts to the batter and beat for 1 minute at medium speed. Turn the batter into a well-greased, lightly floured, 13 x 9 x 2-inch pan.
Bake in a preheated, 350°F oven for about 40 minutes or until the cake tests done. Cool on a rack for 15 minutes, remove cake from pan and turn right-side up on a rack to finish cooling.
In the top of a double boiler, combine all the ingredients except the vanilla extract. Beat 1 minute at high speed of an electric mixer. Place over boiling water (the water should not touch the bottom of the top half of the double boiler). Beat on high speed about 7 minutes until the frosting forms peaks when the mixer is raised.
Remove from boiling water (for the smoothest frosting, empty into a large bowl). Add the vanilla extract and continue beating on high speed until thick enough to spread, about 2 minutes. Spread on the sides and top of the cold banana cake.Servings: 8 to 12