To prepare cake, lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 1/2 cup cocoa, baking powder, and salt in a small bowl, stirring with a whisk.
Place granulated sugar and butter in a large bowl; beat with a mixer at medium speed until well blended (about 5 minutes). Add eggs, 1 at a time, beating well after each addition. Beat in 2 teaspoons vanilla. Add flour mixture and 1 1/4 cups milk alternately to sugar mixture, beginning and ending with flour mixture.
Spoon batter into a 12-cup Bundt pan coated with cooking spray. Bake at 325° for 40 minutes or until a wooden pick inserted in cake comes out clean. Cool in pan 10 minutes on a wire rack; remove from pan. Cool completely on wire rack.
To prepare glaze, combine powdered sugar and 3 tablespoons cocoa. Add 2 tablespoons milk and 1/2 teaspoon vanilla; stir with a whisk until smooth. Drizzle over cool cake; sprinkle with pistachios.