1. Preheat oven to 350˚F. Line a 9×13″ baking pan with parchment paper (leave the edges of the parchment paper hanging over the sides so you can easily pull out the brownies once baked), set aside.
2. In a heatproof bowl add chocolate and butter. You can melt the chocolate and butter using a double boiler or by microwaving on medium heat, whisk to combine.
3. In a separate bowl whisk together flour, baking powder and salt, set aside.
4. In a stand mixer beat eggs, sugar and vanilla together until light and fluffy.
5. Once the chocolate mixture has cooled slowly add to your egg and sugar mixture, and beat until well combined.
6. Add your dry ingredients in two batches until just combined. Fold in the chopped walnuts.
7. Pour the brownie mixture into your prepared pan.
8. Bake for approx. 30-35 minutes until the sides pull away and the middle is set. Use a toothpick and insert it into the middle of the brownies, if it comes out clean they are done.
9. Let brownies cool in the pan on a cooling rack.

Make your icing

1. In a large mixing bowl whip butter until pale and fluffy.
2. Add icing sugar, melted chocolate and whipping cream. Whip until light and fluffy.
3. Add more chocolate for a stronger flavour or more whipping cream if your icing is too stiff.
4. Once the brownies are completely cool ice with chocolate buttercream icing. Let the brownies sit for 15-20 minutes before slicing to let the icing set.