Preheat your oven to 350 degrees Fahrenheit. Grease 4 large ramekins with butter and dust the inside evenly with unsweetened cocoa powder to prevent the cakes from sticking.
Add the melted butter to a mixing bowl along with the sugars and whisk together very well.
Add the eggs and vanilla and whisk until well combined.
Add the flour, cocoa powder and salt and use a silicone spatula to stir the mixture together just until the flour disappears and everything is combined.
Divide the batter evenly between the prepared ramekins. They should be about 3/4 full.
Break the chocolate bar into squares and divide the squares evenly between the ramekins, sinking the squares down into the middle of each of the unbaked cakes. Make sure to cover the chocolate pieces with some of the batter.
Bake the cakes for about 20-22 minutes or until the cakes are set.
Let them cool for about 10 minutes in the ramekins before turning them out onto plates. Dust them with powdered sugar and serve with fresh berries, if desired.