Preheat oven to 350 F. Grease and flour two 9-inch round cake pans. In a saucepan, melt butter and chocolate, stirring, over low heat. Combine flour, sugar, cocoa, baking soda and salt in large bowl of electric mixer. Add chocolate mixture, buttermilk and eggs. Beat on low speed for 1 minute, increase speed to high and beat until light and fluffy, about 2 minutes. Divide batter between cake pans. Bake until cakes spring back when gently pressed, 25-30 minutes. Cool in pans 5 minutes; invert onto wire rack. Meanwhile, bring cream just to a boil. Remove from heat and stir in chocolate chips; mix until frosting is smooth and starts to thicken. Place 1 cooled layer on cake plate. Spread with ½ cup frosting. Top with second layer. Frost top and side of cake with remaining frosting.