Preheat oven to 175°C.  Line a 20cm x 20cm cake tin with baking paper.

In a large bowl, whisk together the buckwheat flour, cacao powder, baking powder and salt.

In another bowl, add the sugar and butter, whisk together then add in one egg at a time, whisking between.  Whisk in the vanilla extract.

Pour the wet mixture into the dry mixture and whisk well.  Fold in the chocolate and pistachios gently without over mixing.

Pour into the tin and bake for about 18-20 minutes until a toothpick comes out mostly clean, but still a little fudgy.

Pour over melted chocolate, sprinkle with chopped pistachios and cut into squares.