Preheat oven to 350°F. Spray a 10- or 12-cup bundt cake pan with baking spray. Set aside.
In a large bowl, add the cake mix, pudding mix, sour cream, vegetable oil, eggs, vanilla extract, and water. Beat on medium speed with an electric mixer for 2 minutes. Fold in the chopped pistachios.
Pour the batter into the prepared cake pan. Bake for 45-50 minutes or until a toothpick inserted into the cake comes out clean. Allow to cool in the pan for 20 minutes before turning out onto a wire rack to cool completely.
Whisk together the glaze ingredients. Drizzle over the cooled cake and top with additional chopped pistachios.
