Grease the ramekins with the butter using your fingers or a pastry brush. Sweep the butter in generous upward strokes up the sides of the ramekins and across the base. Fill the ramekins with a tablespoon of cocoa powder then gently turn the ramekin on its edge, and holding it carefully, slowly turn it for the cocoa to coat the buttered sides. Pour out the excess cocoa into another buttered ramekin and repeat with the rest, adding more cocoa when needed. Chill the ramekins in the fridge as you make the fondants.
Preheat the oven to 200°C/180°C fan/400°F. Place a saucepan of water over medium heat and bring to a simmer, then lower the heat to low and place a mixing bowl on top. Add the butter, cut into rough cubes, and the chocolate, broken into pieces. Allow the butter and chocolate to melt, stirring occasionally. Don't let the water boil or it could overheat the chocolate.
Once it is all melted and stirred into a smooth mixture, add 4 tbsp of espresso (one for each fondant). Stir to combine.
In a separate mixing bowl, break in the eggs and add the sugar. Using electric beaters, whisk the mixture for 5 minutes, until it is light, creamy, and moussey, and when you lift the beaters out, it trails a ribbon of mixture that can support its own weight.
Pour the melted chocolate, butter and coffee into the eggs, sprinkle over the tablespoon of flour and add the salt. Fold it all together until the mixture is evenly chocolate brown and moussey.
Divide the fondant mixture between the four ramekins. Place the ramekins on a baking tray and pop them all in the oven to bake for 11-12 minutes, until the fondants are well-risen, springy, with a slight wobble.
Remove them from the oven. They will be extremely fragile, but run a cutlery knife around the inside of each ramekin to loosen the fondant. Place a serving plate on top and turn it all upside down. Ease up the ramekin which will slowly loosen its grip on the chocolate fondant and it will plop out. Repeat with the other fondants and serve immediately with creme fraiche or vanilla ice cream.