Pre-heat your oven to 390f/180c. 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 M&Ms.
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!