Preheat the oven to 170`c. Line a 30cm x 20cm cake tin with lightly buttered greaseproof paper. Put the dark chocolate and butter in a large bowl, place over a pan of simmering water and allow to melt. Sieve the flour and baking powder into a bowl and set aside. Remove the melted chocolate from the heat and stir in the sugar. Add the eggs and vanilla essence. Fold in the flour and pistachios. Pour the mixture into the prepared cake tin. Place in the oven and bake for 40-45 minutes. The top should be firm but the inside should feel soft when cooked. Allow to cool in the tin. Remove the brownies from the tin and cut into triangles. Using green icing, pipe a zigzag from the top to the bottom of the brownie triangles and sprinkle over coloured balls to decorate. Stick in a straw at the bottom of each brownie and serve.