Preheat your oven to 180ºC/160ºC fan and line a 9" square baking tin with parchment paper!
Melt the dark chocolate and unsalted butter together until smooth in the microwave or in a bowl over a pan of simmering water. Leave to cool for a few minutes
Using an electric whisk/stand mixer, whisk the eggs and light brown soft sugar together for a few minutes until the mixture has doubled in volume, is alot paler, and leaves a trail for a few seconds when you lift the whisk out.
Pour the chocolate/butter mix into the whisked sugar and eggs and fold through very carefully.
Add in the flour and cocoa powder and fold through carefully again.
Add in the chocolate chips and fold in and then pour the mixture into the tin!
Bake in the oven for 25-30+ minutes or until there is a very slight wobble in the middle.
Leave to cool in the tin - and then 'set' the brownies in the fridge for 3-4 hours.
Decoration
Once the brownies have cooled and set - beat the unsalted butter on it's own for a few minutes.
Add the icing sugar into the beaten butter and beat for a few minutes to soften and combine.
Put half of the buttercream into another bowl and beat in the cocoa powder until smooth.
Add a couple of spoonfuls of buttercream into another bowl and colour this one green, and colour the rest orange. You should have three different buttercream at this point.
Spread the chocolate buttercream onto the brownies, and press the oreo crumbs into the buttercream quite firmly.
Portion the brownies now - I did 16 squares.
Using whatever piping tips you prefer - I used a jumbo round tip and 2d closed star piping tip, pipe some pumpkins onto each brownie.
Using a small round piping tip with the green food colouring, pipe some tendrils onto the pumpkins as well.
Notes
You can flavour these brownies whatever you want using flavour extracts!