Preheat the oven to 450°F and line a baking sheet with parchment paper.
Whisk together the dry ingredients in a large bowl.
Place the stick of butter in a large, heat-safe measuring cup and microwave in 20-second increments until it is melted.
Pour the milk into the measuring cup with the butter and whisk to combine.
Make a well in the dry ingredients and pour in the milk-butter mixture. Mix with a spoon until everything is evenly combined but not overmixed.
Using a ¼ cup measuring cup, scoop out the biscuit batter onto the prepared baking sheet, leaving a couple inches in between each biscuit. Bake for 10 minutes, or until puffed and golden around the edges. Serve warm with the pistachio butter.
For the Pistachio Butter:
Place the pistachios in the bowl of a food processor and pulse until the pistachios are very finely ground.
Add the powdered sugar and butter to the food processor and pulse a few times to combine. Season, to taste, with salt or more sugar. Serve with the biscuits!