Cake: Heat oven to 325 degrees F. Coat two 9 x 2-inch round baking pans with nonstick flour-and-oil baking spray.

Coarsely chop chocolate. Place in a small microwave-safe bowl with oil. Microwave on HIGH power for 1 minute; stir until smooth. If needed, heat an additional 15 seconds. Let cool slightly.

In medium-size bowl, mix flour, cocoa powder, baking soda and salt. In large bowl, with mixer on medium speed, beat butter and sugar until fluffy, 3 minutes. Add eggs, one at a time, beating after each. Beat in melted chocolate mixture and vanilla.

Place sour cream in a 2-cup capacity measuring cup. Whisk in milk until smooth. On low speed, beat flour mixture into butter mixture, alternating with thinned sour cream, beginning and ending with flour; beat 1 minute. Spread into pans.

Bake at 325 degrees F for 35 to 40 minutes or until toothpick inserted in centers tests clean. Cool cakes in pans on wire racks 15 minutes. Turn cakes out onto racks; cool layers completely (about 2 hours).

Frosting: In large glass bowl, microwave chocolate chips on medium power for 30 seconds; stir. Microwave another 30 seconds until completely smooth. Let cool slightly. With mixer, blend 1-1/2 cups of the sugar with butter. Beat in sour cream, milk and then half of the chocolate until smooth. On low speed, beat in remaining sugar until blended; then beat in remaining chocolate. Continue to beat until good spreading consistency, about 1 minute.

Slice one of the cooled layers in half horizontally (insert toothpicks into sides of layer to use as guide). Repeat with other layer. Gently place one layer, cut-side down, on serving plate (layers will be tender). Spread top with 2/3 cup frosting. Repeat with remaining cake layers and frosting, ending with cake layer, cut-side down. Frost top and side of cake with remaining frosting. Refrigerate 1 hour, until firm.