Preheat oven: 180°C / 160°C (fan) / Gas Mark 4 / 350°F
In a heatproof bowl, melt the butter and dark chocolate either in the microwave (30secs at a time) or over a double boiler, stirring until smooth.
In a separate mixing bowl, using an electric mixer beat the eggs and sugars. This should whip up to thick and creamy like a milkshake and can take roughly 3-8mins.
Pour the cooled chocolate mixture over the eggy mousse and then gently fold together.
In a separate bowl, sieve the plain flour and cocoa powder, whisking together.
Add the wet ingredients into the dry mixture, slowly folding in until only just combined.
Gently stir in the Bailey's Irish cream until only just smooth.
Pour the brownie batter into a greased and lined 8-inch square baking pan and gently smooth the top. Break up the Oreos (roughly into quarters) and press them lightly into the brownie batter.
Bake for 30-35mins, until papery and cracked on top, but with a slight wobble in the middle. Allow the brownie to cool in the tin before removing and cut into squares.