For the puree: Place the raspberries in a blender and blend until smooth (yields 1 1/4 cups of puree). Remove 1/4 cup of puree and reserve.
For the cake: Place an oven rack in the center of the oven. Preheat the oven to 325 degrees F. Butter and flour the bottom and sides of two 8- or 9-inch round nonstick baking pans. Set aside.
Place 1 tablespoon of the cake mix and the chocolate chips in a small bowl. Toss until the chocolate chips are coated.
In a large bowl, combine the remaining cake mix, 1 cup of the raspberry puree, vegetable oil, eggs and 1/4 cup water. Using an electric hand mixer, blend on low speed for 30 seconds. Increase the speed to medium and beat for 2 minutes, scraping the bottom and sides of the bowl with a spatula as needed. Mix in the chocolate chips. Divide the batter between the prepared pans and bake until a cake tester inserted into the center of the cakes comes out clean, 30 to 35 minutes.
Cool the pans on a wire rack for 15 minutes. Remove the cakes from the pans and cool completely on a wire rack, about 1 hour.
For the frosting: In a large bowl, combine the powdered sugar, cocoa powder, butter, cream cheese and sour cream. Using an electric hand mixer, beat on low speed until smooth. Increase the speed to high and beat until light and fluffy, about 1 minute.
In a small bowl, mix together 3/4 cup of the frosting and the reserved 1/4 cup puree until smooth.
To assemble the cake: Place one cake layer, flat side-up, on a cake stand. Using a spatula, spread the raspberry-chocolate frosting over the cake layer leaving a 1/2-inch border. Place the other cake layer on top. Frost the top and sides of the cake with the remaining chocolate frosting. Decorate with fresh raspberries or chocolate curls. Using a damp knife cut the cake into wedges and serve.
Cake can be made 1 day in advance. Refrigerate in an airtight plastic container. Allow the cake to stand at room temperature for 1 hour before serving.
To make chocolate curls, use a vegetable peeler to shave the edges of a bar of room-temperature chocolate. Refrigerate until ready to use.