Put the dark chocolate and butter in a large microwave safe bowl. Microwave in 30 second bursts, stirring between each burst, until melted. Avoid overcooking as this can negatively affect the chocolate.
Add the caster sugar. Stir through with a manual whisk - I say stir as you don’t want to add air but a whisk is still the best tool as it distributes everything quickly.
Add the eggs and vanilla. Stir with the whisk again.
Put the flour through a sieve into the bowl. Stir to combine.
Add the chocolate chips and fold in.
Pour the brownie batter into your prepared tin and level off.
Bake for 30-40 minutes. You can test if it's baked by pushing a skewer into the centre. If just right, you'll see sticky crumbs.
Place the brownie in its tin on a wire rack to cool completely, then chill overnight to get them nice and firm.
Make the frosting
Sift the icing sugar and cocoa powder into a bowl.
Add cubed butter and 2 teaspoon of milk.
Whisk slowly until it starts to clump, then speed up and whip until the frosting is light and creamy. If it seems too thick, you can whisk in more milk, a few drops at a time.
Frost the brownie
Lift the chilled brownie out of its tray and remove the paper. Place on a cutting board.
Trim the edges with a long, sharp knife if you'd like really neat squares (the offcuts are a perfect chef's treat!)
Spread the chocolate frosting thickly onto the chilled brownies.
Cut into 12 equally sized squares in a 3x4 pattern with a sharp knife, wiping between cuts.