1. Heat oven to 325 degrees F. Coat two 9 x 2-inch round baking pans with cooking spray. Line bottoms with waxed paper; coat with spray, flour.
2. In medium-size bowl, mix flour, baking soda and salt. In large bowl, with mixer on medium speed, beat butter, sugar and vanilla until fluffy, 3 minutes. Add eggs, one at a time, beating after each. Beat in chocolate.
3. On low speed, beat flour mixture into butter mixture in 3 additions, alternating with buttermilk, beginning and ending with flour mixture; beat 1 minute. Divide batter between pans.
4. Bake in 325 degrees F oven 35 minutes or until wooden pick inserted in centers tests clean. Cool cakes in pans on wire racks 15 minutes. Turn cakes out onto rack; remove paper; let cool.White Chocolate Frosting:
5. In large glass bowl, microwave white chocolate on medium power for 30 seconds; stir. Microwave another 30 seconds until completely melted and smooth. Let cool slightly. With mixer, beat in sour cream and butter until smooth. Refrigerate 10 minutes. On low speed, beat in sugar until blended; continue to beat until fluffy and good spreading consistency. If too soft, refrigerate another 10 minutes.
6. Place wooden picks around side of one cake, halfway up side. Using picks as guide, slice cake in half horizontally. Repeat with other cake.
7. Place one cake layer, cut side up, on serving plate. Spread top with 1/2 cup frosting. Repeat with remaining cake layers and frosting, ending with cake layer, cut side down. Set aside 1 cup frosting for top. Spread thin coating of frosting around side of cake. Refrigerate 15 minutes, until firm. Decoratively swirl remaining frosting onto side of cake. Frost top with reserved 1 cup frosting. Refrigerate until set, 20 minutes.