Pre-heat your oven to 370℉/180℃. Line a 12 hole muffin pan 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.
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).
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!