1. Preheat oven to 350°F. Pour water and salt into a heavy saucepan. Cut the butter into small pieces and add to water. Heat over low heat, stirring with a wooden spoon, until mixture comes to a boil.
2. Put flour in all at once. Stir until blended and dough forms a ball. Remove dough to a bowl and beat in 3 eggs, one at a time. Continue beating until dough is very shiny. Refrigerate for 30 minutes.
3. Drop large tablespoons of dough 2 inches apart on an ungreased cookie sheet. Brush with beaten egg yolk.
4. Bake 35-40 minutes until puffed and golden brown. Cool puffs on a wire rack. Cut off tops and fill with whipped cream and banana slices. Sprinkle with grated chocolate.