Step 1Sift 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 2Bake 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.)
Step 1Stir sugar and cream in heavy 3-quart saucepan over medium heat until sugar is dissolved, about 3 minutes. Bring to boil, then reduce heat and simmer gently 6 minutes. Remove from heat. Stir in chocolate and butter until smooth. Stir in vanilla.
Step 2Refrigerate until chilled and somewhat set but not solid. Put in processor bowl fitted with metal blade and process briefly until light and fluffy. An alternative: Beat with wooden spoon. Spread on cake.