Pre-heat your oven to 390f/180c Fan. Line 12 muffin holes with cupcake/muffin liners.
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 and cocoa powder, fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined.
Pour about a teaspoon of the batter into each cupcake case, until just covering the base. Add an Oreo and ensure it is flat. Top with remaining batter until cases are 3/4 full (they don't really rise). Sprinkle over some crushed Oreos.
Bake 20 minutes, until shiny and crisp, and barely wobbling. Remove and let cool before eating. For the perfectly Instagrammable photo, use a knife to cut in half!