Pre-heat your oven to 200c/180c Fan. Line 10 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.
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 (check it first, to make sure it isn't so hot that it will cook the eggs) 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).
Bake 20-25 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!