2½ cups cake flour
2½ teaspoons baking powder
½ teaspoon baking soda
½ teaspoon salt
½ cup unsalted butter, softened
1¼ cups granulated sugar
2 large eggs
1½ teaspoon(s) vanilla extract
½ cup buttermilk
1 cup (about 2 bananas) very ripe, mashed bananas
3 cup light brown sugar
1½ cups heavy cream
½ teaspoon lemon juice
5 tablespoons butter, cold, cut into pieces
MAKE THE BATTER: Preheat oven to 350º F. Using a small brush, lightly coat two 9-inch cake pans with softened butter or vegetable-oil cooking spray. Dust with flour, tap out any excess, and set aside. Over a surface covered with a large sheet of waxed or parchment paper, sift the flour, baking powder, baking soda, and salt. Into a medium bowl, resift the flour mixture and set aside. In a large bowl, using a mixer set on medium-high speed, beat the butter until light -- about 1 minute. Add the sugar and continue to beat for 2 more minutes. Add the eggs, one at a time, beating thoroughly after each addition, and mix in the vanilla. Reduce mixer speed to low and add the flour mixture by thirds, alternating with the buttermilk and bananas and ending with the dry ingredients. Mix just enough to blend the batter after each addition.
BAKE THE CAKE: Divide the batter equally between the pans and bake on the middle rack of the oven until a tester inserted into each cake layer comes out clean -- 25 to 30 minutes. Cool in the cake pans on a wire rack for 15 minutes. Using a knife, loosen the cake layers from the pan sides and invert the layers onto the wire rack to cool completely.
ICE THE CAKE: Use a serrated knife to trim the mounded side of the cake layers, if necessary. Line the edges of a cake plate with 3-inch-wide strips of waxed or parchment paper and place a cake layer, trimmed side down, on top. Place 1 cup Caramel Icing on top of the layer and spread evenly. Place the second layer, trimmed side down, on the first and cover the top and sides with the remaining icing.
MAKE THE ICING: In a medium saucepan with a candy thermometer attached, stir sugar, heavy cream, and lemon juice together. Cook the mixture, without stirring, over medium-high heat to soft-ball stage (238º F). Remove from heat and cool the caramel mixture to 140º F. Place the butter on top of the cooled caramel mixture. Remove the thermometer and, using a handheld mixer set on medium-high speed, beat the caramel until it thickens enough to hold its shape, lightens in color, and changes from translucent to opaque -- about 5 minutes. Apply icing immediately