Preheat the oven to 425°F (218°C). Grease two 4-6 ounce ramekins with unsalted butter or baking spray. Then, lightly dust the bottoms and sides of the ramekins with cocoa powder.

Add the butter and chopped semisweet chocolate to a small heatproof bowl. Microwave in 30-second increments, stirring in-between, just until the chocolate and butter is melted. Whisk until smooth and let it cool for a few minutes.

Combine the egg, egg yolk, granulated sugar, and salt in a medium mixing bowl. Using a hand mixer, beat the eggs and sugar until it's thickened and lighter in color, 2-3 minutes. Mix in the vanilla extract.

Add the melted chocolate mixture and all purpose flour to the egg mixture. Use a rubber spatula to gently fold the ingredients together, just until the batter is well-combined and there are no more streaks.

Divide the batter between the two prepared ramekins, and use a rubber spatula to smooth out the tops. Bake the cakes for 11-14 minutes, until the edges of the cakes are set and the centers are still a bit soft. To test if the cakes are ready, gently press the edges of the cakes, they should spring back right away!

Let the cakes set for 1 minute. Then, cover the top of each ramekin with an inverted plate. Using oven mitts, quickly flip the plates so the ramekins are upside down. Gently tap the bottoms of the ramekins, and then use an oven mitt to lift the ramekins and release the cakes. They should pop out easily.

Top the cakes with a scoop of ice cream, a few berries, and/or a dusting of powdered sugar. Serve immediately and enjoy!