Make the Brownie Batter: In a small bowl, break up the dark/semi-sweet chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn't burn. Alternatively, set up a double boiler. Do this by placing a heatproof bowl over a small pot of simmering water. Add the chocolate to the heatproof bowl and stir until melted.
In a large mixing bowl, whisk together melted butter and both sugars. Then add the egg and mix to combine.
Fold in the melted chocolate, followed by the flour and cocoa powder.
Chill the brownie batter in the refrigerator while prepping the croissants.
Line a large baking tray with greaseproof paper.
Slice the four croissants in half along the long side to create an opening.
Scoop the brownie batter into each croissant, then sandwich the croissants closed. Spread a little remaining brownie batter on top of each croissant.
Bake 15-20 minutes until the brownie batter looks set to the touch - the middle will still be fudgy!
Let cool a few minutes before dusting with powdered sugar and serving!