Preheat the oven to 350°F. Line an 8x8 baking pan with aluminum foil and lightly grease. Set aside.
In a large bowl, mix together the butter and cocoa powder. Add in the eggs, sugar and vanilla, and mix until just combined.
In a separate bowl, mix together the flour, salt and baking powder.
Slowly mix in the dry ingredients into the wet ingredients until just combined. Fold in the white chocolate and macadamia nuts.
Pour the batter into the prepared pan and bake in the preheated oven for 25-30 minutes. Allow to cool completely in the pan before cutting into squares.