Prepare 6-8 ramekins by brushing them with the soft butter and then dusting with the cocoa powder. Make sure to tap out all the excess cocoa powder.
Melt the chocolate with the cubed butter in a bain-marie (a heatproof bowl set over a pan of simmering water), stirring occasionally. Alternatively, melt them in the microwave on a low setting.
Combine the brown sugar and flour in a bowl. Mix the melted chocolate with the eggs followed by the flour and sugar. Divide the mixture between the ramekins and refrigerate for a minimum of 1 hour.⏱ 60 min
Preheat the oven to 350°F. Bake the cakes for 15-20 minutes or until the 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.⏱ 20 min
Serve immediately.