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. You are looking thick, pale egg yolks. This can also be done with a hand mixer, gentleman. Whisk in the chocolate/butter mixture until smooth. Reduce the mixer speed to low and stir in vanilla extract. Gently fold the flour into the chocolate/egg mixture until just combined. Do not overmix.
Spoon the cake batter mid-way into the prepared ramekins. Nestle a Nutella filling ball in the center of each ramekin, then add remaining batter to the ramekins. Leave about ¼ inch space from the top of the ramekin so the cake does not spill. Place the cake in the oven and bake for 10-12 minutes until the cakes soufflé, meaning they rise above the top of the ramekins. The top of the cakes should look a bit cracked, but the centers should remain soft.
Let the cakes cool in the ramekins for about 1 minute, then invert onto a dessert plate. Let stand for a few seconds and unmold. Sprinkle with confectioners’ sugar and serve immediately.