Preheat oven to 350°F. Prepare two 6 in. x 2 in. round pans with vegetable pan spray.

In medium bowl, combine flour, cocoa powder baking soda and salt; set aside.

In medium microwaveable bowl, melt chocolate and butter on 50% power for one minute. Stir and continue to melt for 30 second intervals at 50% power until smooth. Transfer mixture to large bowl. Add sugar and beat with and electric mixer on medium speed until well blended. Add eggs and vanilla; beating until combined. Add flour mixture alternately with sour cream; beat until just combined. Divide evenly between prepared pans, filling pans 2/3 full.

Bake 35-38 minutes or until toothpick inserted comes out clean. Cool in pans on cooling grid 10 minutes, then remove and cool completely. Fill and frost with Chocolate Buttercream.