Preheat your oven to 350°F. Butter 3 (9 inch) cake pans and line the bottoms with parchment paper. Lightly dust the sides with flour to prevent sticking and set aside.* Note: If you don't own 3 cake pans, you can bake 2 thicker cakes instead, or bake the cakes in batches. The cake batter can sit out at room temperature for over 30 minutes. Make sure the cake pan is fully cooled and wiped clean before using. Add pistachios to the bowl of a food processor. Pulse the processor on and off until the nuts are finely ground. Take care not to over pulse the pistachios or they will release too much oil and turn into butter! (Optional) After pulsing a few times, take out and set aside a few larger pistachio pieces for added texture. In a medium bowl, combine ground pistachios, flour, baking powder, baking soda, and salt. Mix to combine, and set aside. In a large bowl, combine butter, oil, and sugar. Using an electric mixer fitted with the beater attachment or a whisk by hand, cream together until well combined and lightened in color (about 2 minutes on medium speed or 5 minutes by hand).
Whisk egg whites into the sugar mixture until broken up and combined. Whisk in vanilla extract and sour cream.
Alternate adding dry ingredients and buttermilk to the egg white mixture. Whisk about half of the dry ingredients, followed by all of your buttermilk (optional - for added color, mix in a few drops of food coloring with the buttermilk). Finish off by whisking in the remaining dry ingredients until just combined. Take care not to over mix!
Divide the cake batter evenly between your cake pans. Bake in the center of your preheated 350°F oven for 20 to 25 minutes, or until a skewer inserted into the center of the cake comes out clean.
Cool the cakes in the pans for about 10 minutes, then turn onto a wire rack and set aside to cool completely to room temperature before frosting.
Make your favorite frosting, and layer and decorate your cakes! I used vanilla buttercream and decorated with blackberries and chopped pistachios. But feel free to get creative!