In a small bowl, break up the dark 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. With an electric/stand mixer, go for a solid 3-5minutes so it is totally combined. Add in the eggs, one at a time and mixing in between.
Now, add the melted chocolate slowly and combine once again.
Add the flour, a pinch of salt and cocoa powder, fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Scoop on top of the cookie/Oreo layer in the muffin liners. Fill close to the top, they don't rise much at all.
Bake 20 minutes. Let cool on a wire rack before serving.