Preheat the oven to 425 degrees F. Coat four 6-ounce ramekins with 1 tablespoon vegan butter, then dust them with 1 tablespoon cocoa powder.
Melt the dark chocolate and 1/2 cup vegan butter together over low heat, stirring until glossy. Remove from the heat and cool for 3 minutes.
Beat the eggs, egg yolks, granulated sugar, vanilla, and salt for 2 minutes, or until slightly thickened and pale.
Slowly whisk the melted chocolate into the egg mixture. Fold in the flour and remaining cocoa powder just until no dry streaks remain.
Divide the batter evenly among the prepared ramekins. Bake for 10 to 12 minutes, until the edges are set but the centers still look soft.
Rest the cakes for 1 minute, then carefully invert each one onto a dessert plate. Dust with powdered sugar and serve immediately while the centers are molten.
