Heat oven to 350°F. Grease bottom and sides of 2 (8-inch) round cake pans with shortening; lightly flour. In small bowl, mix flour, baking powder and salt; set aside.
In large bowl, beat ¾ cup butter and the granulated sugar with electric mixer on medium speed, scraping bowl occasionally, about 2 minutes or until fluffy. Beat in eggs, one at a time, just until smooth. Stir in 2 teaspoons vanilla. On low speed, beat flour mixture into sugar mixture alternately with ¾ cup milk, beating after each addition, until smooth. Scrape side of bowl occasionally. Beat in sour cream. Pour evenly into pans. Tap pans on counter 2 to 3 times to eliminate air bubbles from batter.
Bake 30 to 35 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pans to cooling racks. Cool completely, about 1 hour.
In medium bowl, beat 1 ½ cups butter softened, 3 cups powdered sugar, ¾ cup unsweetened baking cocoa, 2 tablespoons milk, and 1 teaspoon vanilla with electric mixer on medium speed until smooth and spreadable. Place first cake layer on plate, and spread 1 cup frosting over top. Place second cake layer on top of first. Frost side and top of cake with remaining frosting. Store loosely covered at room temperature.
