Preheat the oven to 175C and line the bottom and sides of a 9x9in baking tin with baking paper leaving an overhang on all sides. In a medium bowl whisk the flour, baking powder, bicarb and salt together then set aside.
In a large bowl whisk together the melted butter, brown sugars and caster sugar. Add the eggs and vanilla extract, whisking until fully combined. Add the flour mixture into the wet ingredients, stirring until just combined. The batter will be pretty thick, but don't overmix. Chop the pistachios lightly, so they aren't all whole. Then fold in most of the chocolate chips and pistachios into the mixture, leaving just 1 tablespoon of each behind.
Pour the batter into the prepared baking tin, then sprinkle with the reserved chocolate chips and pistachios. Bake for 35 minutes or until light golden brown on top, covering with foil for the last 15 minutes to prevent the pistachios on top from burning. Allow the blondies to cool completely in the tin on a wire rack.
For the decoration: Melt the 50g white chocolate over a bain-marie, and set aside to cool slightly. Meanwhile, chop the 15g pistachios into a fine crumb and sprinkle over the completely cooled blondies. Then pour the white chocolate into a piping bag, and with the tip cut off drizzle over the blondies. Finally, slice into rectangles.