Preheat oven to 350F and line a brownie tray or 8 inch square cake tin, with baking parchment.
Add unsalted butter and 100g of chocolate to a small heatproof bowl, place over a saucepan of simmering water. Make sure that the bowl does not touch the water. Heat until all the butter and chocolate have melted. Alternatively melt in the microwave. Set aside to cool.
In a large mixing bowl, add caster sugar, eggs and vanilla extract, whisk together.
Add melted butter and chocolate to the whisked egg mixture, and whisk well.
Sift flour and cocoa powder to the chocolate mixture and fold in until well combined.
Fold in chocolate chips.
Pour brownie mixture into lined baking tin and spread evenly with a palette knife.
Bake for 25 minutes, or until a toothpick inserted comes out with a crumb. This should be moist but not sticky.
Leave to cool completely in the tin before frosting and cutting.
For the Frosting
In a large bowl or stand mixer, beat the butter until soft. Add half of the icing sugar and 1 tablespoon of whole milk. Beat until smooth.
Add the remaining icing sugar, milk, peppermint extract and green food colouring, to the desired colour. Beat until smooth.
If the buttercream is too stiff, add a tbsp of whole milk and beat until smooth.
Add a layer of frosting on top of the brownie, smooth using a large palette knife.
For the Ganache
Chop 100g chocolate into small pieces and add to a medium bowl
Add double cream and caster sugar into a small saucepan, and gently heat until beginning to boil.
Remove from heat and pour warm cream over the chocolate, allowing the heat from the cream to melt the chocolate. Stir until all chocolate is melted.
Set ganache aside to cool for 20 minutes, then pour over frosted brownie and sprinkle with chocolate chips.