Bring a small pan of water to a boil over medium heat. Put a larger heat-resistant glass bowl on top. Break or chop up the dark chocolate into smaller pieces. Add both chocolate and butter to the bowl.
Let them both melt over the boiling water bath (bain marie). Stir the melted butter and chocolate well until you get a glossy chocolate mix. Pour in the strong espresso and add the brown sugar as well. Season with a pinch of salt.