Preheat the oven to 180°C/350°F/gas 4. Line a 20cm rectangular baking tin.
Break up the chocolate into a heatproof bowl and add the chopped butter. Place over a pot of simmering water and allow it to melt together with an occasional stir. When melted allow remove from heat and allow to cool slightly.
While the chocolate and butter are melting in a large bowl add the cocoa powder, flour, backing powder, nuts, sugar and orange zest bowl, add the baking powder and sugar, then mix together.
Make a well in the centre of your dry mix and add the eggs. Lightly mix into the mix.Add the slightly cooled butter and chocolate mixture and mix until all is well combined and looks silky and shiny
Pour the mixture into your backing tin ensuring it is evenly spread.
Bake for 25 minutes for a gooey centre and 30 minutes for a firmer centre.