brown sugar, firmly packed
In a mixing bowl, cream shortening and sugar; beat in egg and egg white.
Add buttermilk and bananas; mix well.
Combine flour, baking soda and salt; stir into the creamed mixture.
Add vanilla and nuts; pour into two greased and floured 9" round baking pans.
Bake at 350 degrees for 35 minutes or until cake tests done; cool in pans 10 minutes before removing to a wire rack.
For filling, melt butter and brown sugar in a saucepan over medium heat.
In a small bowl, combine flour and salt with a small amount of milk; stir until smooth.
Add the remaining milk gradually.
Add egg yolk and mix well; stir into saucepan.
Cook and stir over medium heat until very thick, about 10 minutes; add vanilla and nuts, then cool.
Spread between cake layers; dust with confectioner's sugar and chill.
Store in the refrigerator.