Sift first four dry ingredients together.
Combine milk and lemon juice and set aside to sour.
Cream shortening, add sugar gradually, and beat until fluffy.
Beat in eggs, one at a time.
Add vanilla.
Then add dry ingredients alternately with pawpaw puree and soured milk.
Fold in the beaten egg whites and the chopped nuts.
Pour into two lightly greased and floured 9-inch layer-cake pans.
Bake in a moderate oven (350°F) 35 to 40 minutes.
For frosting, cream the butter until fluffy, using an electric mixer.
Blend in the lemon juice and a small amount of grated lemon rind.
Add the confectioners’ sugar gradually along with enough cream to make a frosting of the right spreading consistency.
Run the beaters long enough to make the frosting very fluffy.
Garnish the top of the frosted cake with a grating of lemon rind.
