Prepare 6 to 8 ramekins by brushing them with soft butter and then dusting with the cocoa powder. Make sure to tap out all the excess cocoa powder.
Melt chocolate with cubed butter in a heat-proof bowl set over a pan of simmering water, stirring occasionally. Alternatively, melt chocolate and butter in microwave on low.
Combine brown sugar and flour in bowl. Mix melted chocolate with eggs, followed by flour and sugar, Divide mixture between ramekins 3/4 full. Preheat oven to 350 degrees.
Make caramel filling: Sprinkle a thin layer of sugar over bottom of a heavy-bottomed pan and place on medium heat. Once sugar starts to melt, add some more sugar. Repeat a couple of times until all the sugar has melted. Continue heating the caramel, swirling it around in pan (do not stir). When caramel is almost a Coca-Cola color, add cream and salt (be careful as caramel may splatter). Cook until temperature reaches 226 degrees, or until caramel coats the back of a spoon, then pour into a dish and leave to cool a little.
Once caramel is cool, transfer to a piping bag fitted with a small round nozzle, or to a heavy-duty food bag (just snip off corner to use). Pop the nozzle into the middle of the chocolate mixture in each ramekin and squirt in the filling. The mixture will rise almost to the top.
Bake in preheated oven for 15 to 20 minutes, or until edges are firm and the centers slightly runny. Test by inserting a toothpick in the center; it should come out wet. Leave to rest for 2 minutes before turning the cakes out of the ramekins onto plates. Serve immediately.