Preheat the oven to 450°.
In a stand mixer with the whisk attachment on high speed, whisk together the 3 large eggs, ½ cup sugar, pinch of sea salt, and 1 teaspoon vanilla until it turns white and is light and fluffy, about 3 to 4 minutes. Set aside.
Bring a medium-sized pot a third filled with water to a low simmer over low-medium heat. Add the 10 tablespoons unsalted butter and 8 ounces of finely chopped bittersweet chocolate in a metal bowl and place it over the pot. Stir constantly using a rubber spatula until completely melted and smooth.
Pour about a ½ cup of the melted chocolate into the whisked egg and sugar mixture and fold together using a rubber spatula for 20 to 30 seconds to temper the eggs. Pour in the remaining chocolate and fold until completely mixed in.
Fold in the ¼ cup all-purpose flour using a rubber spatula until mixed in.
Transfer the batter into buttered and floured ramekins or spread it out on a 13×9 casserole dish. Place in the refrigerator until completely cool, about 30 to 45 minutes.
Butter and flour 6 8-ounce ramekins or 6 cylinders, and place on a cookie sheet tray lined with parchment paper.
Scoop out the batter and evenly distribute it across the ramekins or cylinders.
Bake in the oven at 450° for 9 minutes on the dot.
If using a ramekin, invert on a plate for 10 seconds and remove, or pull the cylinder up and use a spatula to transfer it to a plate.
Garnish with optional whipped cream, chocolate sauce, strawberries, or ice cream, and serve.