In a double boiler set over medium-low heat, melt 160 ml chocolate chips with 80 ml butter.
Using a mixer, beat the sugar and eggs until creamy and smooth. Fold in the melted chocolate chip mixture.
Add the lemon zest, vanilla extract and stir. Alternately add the flour and milk, stirring between each addition.
Once the mixture is smooth, add the white chocolate chips and pistachios. Pour the mixture into a rimmed 9 in. x 13 in. dish, greased and lined with parchment paper. Bake in the oven for 25 minutes.
If desired, melt the chocolate chips and vegetable oil in a double boiler. Once the blondies have cooled, drizzle with the melted chocolate topping.