01 - Preheat oven to 350°F. Grease and line two 8-inch round cake pans with parchment paper.
02 - In a food processor, finely grind 1 1/2 cups pistachios. Set aside.
03 - Whisk together flour, baking powder, and salt in a bowl.
04 - Beat butter and sugar together until light and fluffy, about 3 minutes.
05 - Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
06 - Mix in the ground pistachios until evenly distributed.
07 - Alternately add flour mixture and milk, beginning and ending with flour. Add sour cream and mix until just combined.
08 - Divide batter evenly between prepared pans. Bake for 28-32 minutes, or until a toothpick inserted in center comes out clean.
09 - Cool cakes in pans for 10 minutes, then transfer to wire rack to cool completely.
10 - Process 1 cup pistachios until very finely ground or beginning to form a paste.
11 - Beat butter until creamy. Add pistachio paste and mix well.
12 - Gradually add powdered sugar, beating until fluffy. Add vanilla and salt. Add cream 1 tablespoon at a time until desired consistency is reached.
13 - Once cakes are cooled, spread frosting between layers and over top and sides. Decorate with extra chopped pistachios if desired.