Preheat oven to 375-400 degrees F.

Grease the bottom and sides of six 6- or 8-ounce ramekins by brushing or spreading a thin layer of butter evenly over the bottom and sides of each ramekin, paying close attention to the crevices, especially where the sides and bottom meet. Sprinkle in cocoa powder to coat evenly and tap out the excess.

In a medium bowl, add the chocolate and butter and melt in 1-minute increments in the microwave, stirring in between, until melted and smooth. Off the heat, stir in the salt and vanilla. Let the mixture cool to room temperature.

In a large bowl with a handheld electric mixer or in the bowl of an electric stand mixer fitted with the paddle attachment, beat the eggs and egg yolks together until thick and light in color, 2-3 minutes.

Add the powdered sugar and mix until combined. Add the cooled, melted chocolate mixture and the flour and mix with a spoon or spatula just until combined and no streaks remain.

Divide the batter evenly among the ramekins and place the ramekins on a large baking sheet with a couple inches between each dish.

Bake for 12-13 minutes until the sides are set and the cakes feel soft when touched in the center.

Remove from the oven and let sit for 2-3 minutes. Run a small knife or metal spatula around the edges to loosen. Place a plate upside down on the ramekin, and holding onto both the plate and the ramekin, carefully flip so that the cake falls out onto the plate.

Dust the cakes with powdered sugar or serve with whipped cream and strawberries or ice cream and hot fudge sauce.