For the brownies:
170g unsalted butter, softened
200g dark brown soft sugar
100g caster sugar
1 tablespoon vanilla extract
1 egg + 1 egg yolk
250g plain flour
1/2 tsp bicarbonate of soda
100g chopped mixed nuts
For the frosting:
70g butter, softened
140g icing sugar
100g milk chocolate, melted
Preheat your oven to 180c and line a deep sided baking tray with greaseproof paper.
In a large bowl, cream together the butter and sugars until light and fluffy. Mix in the vanilla, egg and egg yolk until smooth.
Sift in the flour and bicarbonate of soda and fold this into the mixture. Lastly, add the Nutella and chopped nuts and give it a good mix.
Tip the mix into your prepared baking tray and push down with your hands, spreading it up to the edges and into the corners, leaving a level surface.
Bake for 15 to 17 minutes, until it starts to go a little firm around the edges, but the middle is still soft. Leave to cool in the tray.
While it's cooling, make the frosting. In a clean bowl, cream together the butter and icing sugar until you have a fluffy buttercream. Pour the melted chocolate into the butter cream and carefully mix it in.
Pour the frosting over the cooled brownies and spread with a palette knife to cover the whole surface. Leave to set in the tray before removing and cutting into squares.