For the filling/topping:
[Food Editor's note: I usually double the cream and use whatever looks right to me, saving the rest in the fridge for future desserts!]
1 cup heavy cream
1 1/2 tablespoons sugar
Preheat oven to 350 degrees. Grease two 9-inch round pans.
Sift together flour, baking powder, baking soda, and salt. Set aside.
Beat egg whites until soft peaks form. Continue beating while adding 3 tablespoons sugar taken from amount in recipe. When stiff peaks form, stop beating and set aside.
In a large mixing bowl, cream the butter and add the rest of the sugar gradually while continuing to beat. Add the mashed bananas and beat one minute. Add egg yolks one at a time and then the vanilla. Beat on low while adding flour mixture alternately with the sour cream mixture. Do not overbeat. Fold in the egg whites until evenly distributed.
Divide the batter into the two pans and bake about 30 minutes, until a tester comes out clean. Cool for a while in the pans and then turn out onto racks to cool completely.
Whip the cream until fluffy and thick, adding the sugar very gradually. Slice one of the bananas. Spread a "good" amount of the whipped cream on the bottom layer and cover with sliced bananas. (Slice more if necessary.) Place the second layer on top and spread with whipped cream. If not serving immediately, place the cake in the refrigerator. (Remove 15-30 minutes before serving, to take off the chill.)
Just before serving, slice the remaining bananas and put them in a ring around the edge of the top of the cake.
Refrigerate any leftovers.
Paulette Idelson lives in Newton and is the Food Editor's aunt.
Banana Layer Cake