Preheat the oven to 350°F (175°C or 165°C for a fan oven).
Grease a 13 x 9 x 2 inch (33 x 22 x 5 cm) pan.
Sift the cocoa and baking soda into a large bowl.
Stir in ⅓ cup of the butter. (That's 75 grams or half of the butter)
Add the boiling water and stir until the mixture thickens up.
Stir in the sugar, beaten eggs, vanilla and the remaining ⅓ cup butter.
Stir well until the mixture is smooth.
Sift in the flour and salt.
Stir until well blended.
Stir in the mint chips.
Spread the batter in the prepared pan.
Bake 35 to 40 minutes or until the brownies begin to pull away from the sides of the pan.
Place the pan on a wire rack to cool.
Meanwhile, put the icing sugar in a bowl and gradually add the water until you have a smooth, drizzle-able glaze.
Stir in the mint flavouring and a drop of green food colouring.
When the brownies are completely cool, drizzle with the glaze.
Let the glaze set for at least a half hour before cutting the brownies into squares.