Coat a 9-inch bundt pan with butter or margarine. Preheat the oven to 350°F (175°C).
Whisk the sugar with the eggs, adding one egg at a time. Incorporate the butter, then use an electric mixer to beat the mixture for about 3 minutes until it becomes light and creamy.
In a separate bowl, combine the baking powder and flour. Soak the pistachios and hazelnuts in water for a minute, then drain and stir them into the flour mixture (set aside about 2 tbsp of pistachios for garnish). This will help keep the nuts from sinking to the bottom of the cake.
Gradually fold the flour into the batter, mixing gently until fully combined.
Pour the batter into the bundt pan and bake for approximately 40 minutes. Allow the cake to cool before removing it from the pan.
Glaze
In a small bowl, mix the pistachio instant pudding with 1 ½ cups of milk. Once it has set, spread it over the top of the bundt cake and garnish with the remaining pistachios.