Preheat the oven to 375°Fahrenheit. Butter 4, 4 oz ramekins and sprinkle with cocoa powder. Move the ramekin around to distribute the cocoa powder evenly. Tap out the excess cocoa powder and place the ramekins on a baking sheet. Set aside.
Melt the chocolate:
Place a metal bowl over a sauce pan with simmering water. Add the butter and the chocolate, stirring occasionally until completely melted. Set aside to cool.
Make the cake:
In a mixing bowl fitted with a whisk attachment, beat the eggs, yolks, sugar and salt at high speed until very thickened, about 2 minutes. What you’re looking for are pale, thick egg yolks. Whisk in the chocolate/butter until completely smooth and then stir in vanilla extract. With a spatula, gently fold in the flour into the chocolate mixture until just combined.
Spoon the cake batter into the prepared ramekins. Leave about 1/4 inch space so the cake doesn’t spill over upon rising. Bake the cake for 10-12 minutes. The cake should have raised much like a soufflé. The sides should be firm; the very center should be soft. Make the topping while the cakes have about 3-4 minutes left in the oven.
Make the topping:
In a medium pan over low/medium heat, melt the dulce de leche slightly. Add the walnuts and vanilla extract and allow the dulce de leche to melt and bubble slightly. Add the sliced bananas and heavy cream. Stir quickly, making sure the walnuts and bananas are completely coated with the dulce de leche. Remove from heat and set aside.
Remove the cakes from the oven and let stand for about 2 minutes. Gently run the tip of a knife around the ramekin to help loosen the cake, and then invert each cake onto a dessert plate. Unmold and top with the banana walnut dulce de leche. Serve immediately.