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.