Preheat the oven to 170°C/150°C/Gas Mark 3. Grease and line a rectangular baking tray roughly 25x18cm.
Break the Chocolate into squares and place in a microwavable bowl, add the butter and microwave for 1 minute. Stir and place back in the microwave heating in 10 second bursts, stirring between each burst until the butter and chocolate has melted.
Stir the sugar into the melted mixture and leave to cool for 10 minutes.
Once cooled add the beaten eggs and Vanilla extract, mix in to create a smooth, glossy mixture.
Sieve the flour, Baking Powder and Cocoa Powder on top and gently fold into the mixture. Finally add the Chocolate Chunks and stir through.
Place the mixture into the prepared baking tray and smooth to evenly cover the tin. Place in the oven and bake 25 minutes until the centre of the brownie no longer wobbles and the edges of the brownies are firm to touch. Leave to cool in the tin, this allows the brownie to firm up.
Once cooled spoon the Chocolate Buttercream on top of the brownie and spread to cover the top of the brownie.
Sprinkle the crushed biscuits over the top of the buttercream and place your chosen treats on top of the crushed biscuits. Your Garden Traybake is now ready to enjoy!