Preheat the oven to 350 degrees Fahrenheit. Spray two 8-inch cake pans with pan spray and set aside.
If you don’t have ground pistachios, pulse shelled unsalted pistachios in a blender or food processor until the nuts before a powder.
In a large bowl, whisk together the flour, ground pistachios, sugar, baking powder, baking soda and salt.
In a medium bowl whisk together the buttermilk, milk, both extracts and oil. Add the wet ingredients to the dry and mix until just combined. If desired, add a drop or two of green food coloring for the signature green pistachio color.
In a clean large bowl, whip the egg whites until medium stiff peaks form.
Gently fold the egg whites into the batter until no white streaks of egg whites remain. Separate the batter evenly into the two prepared pans.
Bake for 30-35 minutes, until the tops of the cakes spring back when gently pressed or a toothpick comes out with minimal crumbs on it.
Let the cakes cool in the pans for 4-5 minutes before inverting onto a cooling rack to finish cooling.
Frost the cooled cakes with buttercream and top with chopped pistachios.
Beat the butter and powdered sugar together slowly in a stand mixer until combined. Add the vanilla, salt and 3 tablespoons of cream. Beat until smooth. Add more cream if needed to reach a softer, smoother consistency.