Preheat oven to 400°F.
Use approximately 1 Tablespoon of the melted butter to brush the inside of six, 4-ounce ramekins or custard cups; set aside.
In a large bowl, beat together remaining melted butter, cocoa powder, sugars, flour and salt. Stir in eggs until smooth. Stir in vanilla. Pour batter into the prepared ramekins and set the ramekins in a large baking dish.
Pour hot water into the baking dish to a level about halfway up the side of the ramekins. Bake for 14-15 minutes until the batter puffs but the center is not set. The edges will be firm but the center will be runny.
Serve the cakes in the ramekins or run a knife around the edge of each cake and unmold onto plates. Serve the cakes warm or chilled. Garnish with raspberry sauce, fresh berries, vanilla ice-cream or a dusting of powdered sugar.