Preheat the oven to 400 degrees F. Line 2 baking sheets with parchment paper. Fit a large pastry bag with a 1/2 -inch round tip.
Combine the butter, granulated sugar, salt and 1 cup water in a medium saucepan and bring to a boil. Remove from the heat; sift the flour into the saucepan and stir with a wooden spoon to make a paste. Return to medium heat and stir until the dough pulls away from the side of the pan and is slightly shiny, about 2 minutes. Transfer to a bowl and beat with a wooden spoon until slightly cooled, 2 to 3 minutes. Add the whole eggs, one at a time, beating until smooth before adding the next.
Transfer the dough to the pastry bag. Pipe 1-inch mounds of dough 1 inch apart on the prepared baking sheets. Smooth the tops with a damp fingertip. Beat the egg yolk with 1 tablespoon water; brush on the dough mounds. Bake 15 minutes, then reduce the oven temperature to 350 degrees F and continue baking until the puffs are golden brown and crisp, about 25 more minutes. Transfer to a rack and let cool completely.
Meanwhile, beat the heavy cream and the confectioners' sugar in a large bowl with a mixer until soft peaks form; refrigerate until ready to assemble. Place the chocolate in a microwave-safe bowl and microwave in 30-second intervals, stirring, until melted.
Cut the puffs in half crosswise with a serrated knife; sandwich with the sliced bananas, dulce de leche and whipped cream. Drizzle the puffs with the melted chocolate and more dulce de leche.
Photograph by Ralph Smith
Recipe courtesy of Food Network Magazine