Heat whipping cream and 2 teaspoons lemon rind in a large saucepan over medium-low heat until hot. Add morsels, and cook 5 minutes or until chocolate is melted, stirring constantly. Cool; cover and chill at least 5 hours.
Meanwhile, melt unsweetened chocolate and 1 tablespoon lemon rind in a small saucepan; let cool.
Stir together flour and next 3 ingredients; set aside.
Beat butter at medium speed with an electric mixer until creamy; gradually add sugar, and beat until light and fluffy. Add eggs, 1 at a time, beating just until blended; stir in vanilla. Add flour mixture alternately with yogurt, beginning and ending with flour mixture; stir in melted unsweetened chocolate mixture just until blended. Divide batter evenly among 3 lightly greased 9-inch round cakepans.
Bake at 350° for 25 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes; remove from pans, and cool completely on wire racks.
Meanwhile, beat chilled whipping cream mixture at medium speed until stiff peaks form. (Be careful not to overbeat.)
Spread frosting between layers and on top and sides of cake. Store in refrigerator.