Pre-heat oven: 180°C / 160°C (fan) / Gas 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 sugar. 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, add in the vanilla extract and then gently fold together.
In a separate bowl, sieve the plain flour and cocoa powder, whisking together.
Add the dry ingredients into the wet mixture, slowly folding in until only just combined.
Pour half the brownie batter into a lined 8-inch square baking pan.
Set aside 6 After Eights and place the rest in an even layer on top of the brownie batter half.
Pour the remaining brownie batter on top and gently smooth.
Bake for 30-35mins, until papery and cracked on top, but with a slight wobble in the middle.
As soon as you remove the brownie from the oven, break the remaining 6 After Eights into roughly 4 pieces each and space them out on top of the hot brownie, so they melt slightly.
Allow the topped brownie to cool in the tin before removing, then cut into squares and enjoy!