Preheat oven to 375 degrees. Place puff pastry rounds, 2 inches apart, on a parchment-lined baking sheet and bake until golden and puffed, 15 to 20 minutes. Transfer to a wire rack and let cool.
Meanwhile, in a medium pot, combine corn syrup, cream, butter, sugar, cocoa powder, and pinch of salt; bring to a boil, whisking, over medium-high. Remove fudge sauce from heat and let cool slightly. (To store, refrigerate in an airtight container, up to 2 months.) Top each pastry with bananas, ice cream, sauce, and walnuts.