Step 1 Sift flour, cocoa, baking soda and salt into large mixer bowl; stir in sugar. Add sour cream, butter, eggs and vanilla. Use mixer on low speed to combine ingredients, then increase to medium speed and mix 3 minutes. Grease 13x9-inch baking pan and dust lightly with flour. Transfer batter to pan; use spatula to smooth surface.
Step 2 Bake at 350 degrees until toothpick inserted into center comes out clean, about 40 minutes. Cool completely on wire rack. Spread with frosting. (Can be made day ahead and kept at room temperature, covered airtight.)