Preheat your oven to 450 degrees. Generously butter 2 ramekins (that hold at least 1/2 cup) with 1 tbs of butter, and dust with 1 tbs. of cocoa powder to fully coat the dishes. Set aside onto a small baking sheet for easy transfer to and from the oven.

In a medium bowl, melt the remaining 3 tbs of butter in the microwave until just melted, about 30-60 seconds. Add in the chopped chocolate and stir to melt the chocolate completely. Stir in the salt, flour, and remaining 1 tbs of cocoa powder.

In a another medium bowl, whisk together the egg yolk, egg, sugar, vanilla extract, and salt until the mixture is very pale and thick, about 3-5 minutes. It should form thick ribbons when you hold the whisk above your bowl.

Pour the egg mixture into the chocolate mixture and gently fold to combine until no streaks remain in the batter.

Divide the mixture evenly between your prepared ramekin dishes. Bake for 5-7 minutes, or until the cakes have risen but are still slightly wobbly in the centers. You want the centers to just jiggle slightly when you shake it, but the cake should look set around the edges. Carefully flip out onto plates immediately and serve while warm with whipped cream or ice cream on top!