In a large bowl, whisk together the butter and sugar until combined. Add in the dry ingredients and the eggs, mixing until the batter is just combined. Whisk in the vanilla extract, then stir in the milk until the batter is smooth. Use a spoon and add the batter to the donut pan until each mold is about 3/4 of the way full. Bake the donuts for 15 to 17 minutes, until they are set and slightly golden. Remove and let cool. Once cool, dip each donut in the ganache and sprinkle with chopped pistachios. Serve immediately.