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