Pre-heat oven to 180ºC. Grease and line a deep 20cm square baking tin with baking paper/parchment. (I used round baking tin instead coz I have no available square baking tin.)
Combine the butter and dark chocolate in a heatproof bowl and melt together over a pan of simmering water, stirring constantly until thick. Leave to cool slightly and set aside.
In a separate mixing bowl, whisk together the eggs, sugar, olive oil and vanilla extract, beating until well blended. Add chocolate mixture to egg mixture. Combine flour, cocoa and baking powder, and salt. Mix well.
Fold in the pistachios and white chocolate. Mix until well incorporated but do not over mix. Pour the mixture into prepared baking tin.
Bake for 30 minutes until nicely crusted done. Leave to cool completely until firm and cut into squares.
