In the bowl of an electric mixer fitted with a paddle attachment, whip the butter until light and flu!y, about 4 minutes. Stop the mixer and add the pistachio mix. Mix on low speed until incorporated, then increase the speed to medium. Add the eggs, one at a time, mixing until fully combined between each. Stop and scrape down the bowl if needed. The batter should be light and flu!y. Transfer to the prepared cake pan and bake until set, about 20 minutes. Remove from the oven and cool.