Preheat the oven to 180°C (350°F) and line a 9"x9" baking tin with baking paper.
Melt the butter and dark chocolate together.
Mix the eggs, sugar, brown sugar, vanilla extract and espresso together until the mixture becomes a light golden colour.
Mix in the chocolate mixture.
Stir in the flour and cocoa until just combined.
Mix in the chocolate chips.
Pour the brownie mixture and bake for 20-25 minutes. The top and sides will be set and the middle will have crumbs attached to a skewer inserted but should not be wet.
Allow to cool to room temperature before topping with the buttercream.
Dissolve the espresso in the water.Beat the butter and Nutella together.With the beaters on, add the icing sugar slowly until it is all combined. (How much you add is up to you and how sweet you want the buttercream).If the mixture is too tough, add a little of the milk.Pipe onto the the brownies and slice.