2 chocolate fondant molds, 3 inches in diameter and 2 inches deep
For the salted caramel: Heat the sugar in a medium dry frying pan over medium heat, without stirring but shaking the pan intermittently to disperse some of the sugar, until fully melted and a dark golden brown, 2 to 3 minutes.
Take the pan immediately off the heat and whisk in the salt and half of the cream; the mixture will bubble up very quickly. Add the other half of the cream once it has stopped bubbling so vigorously. Once all the cream is added and it has stopped bubbling, whisk in the butter. Place the pan over medium-low heat and cook, whisking, until the mixture has thickened slightly, another 2 minutes.
Pour the hot caramel into an ice cube tray and place in the freezer for 3 hours or overnight. You should be able to make between 8 and 10 caramels roughly 1/2 inch in size, depending on your ice cube tray. The salted caramel cubes can also be kept in the freezer for 2 weeks for later use.
For the chocolate cake: Preheat the oven to 200 degrees C/390 degrees F.
In a medium bowl, beat the egg and sugar with a hand whisk until the mixture appears light and fluffy, 1 to 2 minutes.
Put the chocolate and butter in a heatproof bowl and place over a double boiler, stirring occasionally, until completely melted, 1 to 2 minutes. Remove the bowl of chocolate from the heat and slowly pour over the egg mixture. Whisk until combined. Sift in the flour and stir the mixture together gently with a spatula until all of the flour is incorporated.
Using some spare soft butter, grease the inside of 2 chocolate fondant molds (roughly 3 inches in diameter and 2 inches deep). Place the cocoa powder into the bottom of one mold. Rotate the mold, tipping the excess cocoa powder around the edges until the mold is completely coated. Tip the excess cocoa powder into the second mold and repeat the process. Tip out any remaining cocoa powder from the mold once finished.
Using a tablespoon, fill the molds up two-thirds of the way with the chocolate mixture. Remove 2 caramel cubes from the freezer and push a cube into the center of each cake, making sure the caramel is covered by the chocolate batter.
Bake until the tops have formed a slight crust and the cakes start to pull away from the sides of the molds, about 11 minutes. Immediately remove the cakes from the oven. Place a serving plate on top of each mold and flip it over, releasing the cakes. Serve immediately with cream or ice cream.