Preheat the oven to 450°F. Butter and lightly flour two 6-ounce ramekins. Set the prepared ramekins on a baking sheet.

In a microwavable bowl, combine finely chopped chocolate and cubes of butter. Microwave for 30 seconds, stir. Heat in 15 seconds increments, stirring in between each one, until mixture is completely melted and smooth. Let cool for about 5 minutes.

In a medium bowl, beat the egg with the egg yolk, sugar and salt at high speed until thick and pale.

Add the chocolate and flour to the egg mixture and mix until combined.

Divide the batter between the prepared ramekins. If you're using dulce de leche, fill the ramekin halfway, and then add 1 tablespoon of dulce de leche to each. Top with the remaining batter.

Bake for about 9-10 minutes, or until the sides of the cakes are firm but the centers are soft. Let the cakes cool for 1 minute, run a thin knife around each to loosen the cake, and then cover with an inverted dessert plate. Carefully turn each one over, let stand for 10 seconds and remove the ramekin.

Garnish with powdered sugar and berries if desired. Serve immediately.