Preheat the oven to 350 degrees F. Butter and lightly flour a 9-inch cake pan. Cream the butter and slowly add the sugar, beating until light. Add banana, eggs and vanilla and beat well. Mix the flour, salt, and baking soda, add to the first mixture and blend. Slowly add the sour milk or cream, and beat until well blended Spread in the pan and bake for about 45 minutes, or until a toothpick comes out clean. Cool in the pan for 5 about minutes before turning out onto a rack. Split the cake, fill and frost with Banana Cream Filling and coat with crushed nuts (optional). For Banana Cream Filling: Heat the milk in a heavy bottomed pan until very hot but not boiling. Mix the sugar, flour and salt together in a bowl. Stir in the hot milk and beat until well blended. Pour back into the pan and continue to stir vigorously over low heat for minutes, until very thick and smooth. Add the egg yolks and cook for a few more minutes. Cool, stirring from time to time. Mash the banana and beat it until smooth. Add 2 tablespoons lemon juice, 1 teaspoon vanilla (optional), and stir the mixture into the cooled filling.