PREHEAT oven to 350° F. Grease and flour three 9-inch round cake pans.
COMBINE cake mix, sour cream, water, eggs, vegetable oil and vanilla extract in large mixer bowl. Beat on low speed for 30 seconds. Beat on medium speed for 2 minutes more. Stir in 1 cup mini morsels. Divide batter among the prepared pans.
BAKE for 20 to 25 minutes or until a wooden toothpick inserted near the centers comes out clean. Cool in pans on wire racks for 10 minutes. Remove cake layers from pans to wire rack; cool completely on wire racks.
COMBINE 2 cups semi-sweet morsels, 1 cup whipping cream and the butter in a medium saucepan over medium low heat until melted and smooth. Transfer to medium bowl. Cover; refrigerate for 1 hour. Strain raspberries and discard the seeds. Combine the strained raspberry mixture, sugar and cornstarch in a small saucepan. Cook and stir over medium heat until mixture is thickened and bubbly. Cook and stir 2 minutes more. Transfer to small bowl. Cover; refrigerate for 1 hour.
BEAT the remaining whipping cream until soft peaks form in small mixer bowl. Fold in chilled raspberry mixture. Stir the chocolate mixture with a wooden spoon until mixture reaches desired spreading consistency.
To assemble, place 1 cake layer on a serving plate. Spread half of the raspberry mixture over the top to within 1 inch of the edge of the cake. Top with a second cake layer. Spread with remaining raspberry filling to within 1 inch of the edge. Top with remaining cake layer. Frost top and sides of cake with chocolate mixture, spreading smooth. Garnish with fresh raspberries and mint, if desired.