Preheat oven to 350°.
Coat bottoms of 2 (9-inch) round cake pans with cooking spray (do not coat sides of pans). Line bottoms of pans with wax paper. Coat wax paper with cooking spray; dust with 2 teaspoons flour. Set pans aside.
To prepare cake, place 2/3 cup chopped pistachios in a mini food processor; process until a smooth paste forms (about 3 to 4 minutes), scraping down sides occasionally. Reserve 1 tablespoon pistachio butter; set aside.
Place remaining pistachio butter and butter in a large bowl; beat with a mixer at medium speed until creamy. Gradually add 1 cup granulated sugar, beating well. Add 2 egg yolks, one at a time, beating well after each addition.
Lightly spoon 2 cups flour into dry measuring cups; level with a knife. Combine 2 cups flour, baking soda, and 1/4 teaspoon salt, stirring with a whisk. Add flour mixture and buttermilk alternately to butter mixture, beginning and ending with flour mixture. Beat in 1 teaspoon vanilla.
Place cream of tartar and egg whites in a large bowl; beat with a mixer at high speed until stiff peaks form using clean, dry beaters. Gently fold one-third of egg white mixture into batter; gently fold in remaining egg white mixture. Divide batter evenly between prepared pans. Bake at 350° for 24 minutes or until golden brown and a wooden pick inserted in center comes out clean. Cool in pans 5 minutes on a wire rack. Run a knife around edges of pans; loosen cake layers from pans. Remove cakes from pans. Discard wax paper. Cool cake layers completely on a wire rack coated with cooking spray.
To prepare filling, combine 3 tablespoons granulated sugar, cornstarch, and dash of salt in a small, heavy saucepan. Add 1 cup 1% milk and egg yolks, stirring with a whisk. Cook over medium-high heat until mixture comes to a boil, stirring constantly with a whisk. Remove from heat; add reserved 1 tablespoon pistachio butter and 1/2 teaspoon vanilla, stirring with a whisk. Place pan in a large ice-filled bowl for 20 minutes or until egg mixture cools to room temperature, stirring occasionally.
To prepare frosting, place cream cheese in a large bowl; beat with a mixer at high speed until fluffy. Gradually add powdered sugar, beating at low speed just until smooth (do not overbeat). Beat in 1 1/2 tablespoons 1% low-fat milk and 1 teaspoon vanilla until blended and frosting is of spreading consistency.
Place 1 cake layer on a plate; spread the layer evenly with filling. Top with the remaining cake layer. Spread frosting over top and sides of cake. Sprinkle cake with 1 tablespoon chopped unsalted dry-roasted pistachios.