Preheat oven to 325 degrees F. Line the bottom and long sides of a loaf pan with a sling of parchment paper. Coat paper and exposed short sides of loaf pan with nonstick spray or butter.
In a food processor, grind pistachios, sugar and salt together until as powdery as you can get them without it turning to paste. Add butter and blend with pistachio mixture. It will get lumpy, then turn into a ball, but just keep running the machine until the mixture loosens up and it is one consistent mixture. Add eggs, one at time, blending briefly between each, scraping down sides as needed. Add milk, blend to combine. Add vanilla extract and baking powder and blend to fully combine, scraping down bowl. Add flour and tapioca starch, pulse just until it disappears.
Pour batter into prepared pan and spread top smooth. Bake for 50-55 minutes, looking for a toothpick inserted into the center of the cake to come out clean and then, do a second check near the top. Let cake cool in pan on rack for 10 minutes, then run a knife around cake and transfer to cooling rack. Let cool completely.
To make frosting, beat cream cheese and butter together with a hand mixer. Slowly add in powdered sugar and continue beating until fluffy. Spread onto cooled cake, and top with additional crushed pistachios and optionally, edible flowers.
This recipe is adapted from Smitten Kitchen's Pistachio Cake.