Since is says INSTANT pudding, that's what I'd use for best results. Here's the recipe:

Triple Chocolate Pound Cake

serves/makes: 12

Ingredients:

1 package (18-ounce size) devil's food cake mix with pudding
1 package (5.1-ounce size) chocolate instant pudding mix
1 1/4 cup water
1/2 cup vegetable oil
4 large eggs
3 cups semisweet chocolate chips, divided
1/2 cup heavy cream
pecan halves for garnish

Directions:

Preheat oven to 350 degrees.

Grease and flour a 10- by 4-inch Bundt pan.

In a large bowl, combine cake mix, pudding mix, water, oil and eggs. Beat 2 minutes at medium speed with an electric mixer. Stir in 2 cups chocolate chips. Pour batter into prepared pan.

Bake for 50 to 55 minutes. Cool in pan on a wire rack for 10 to 15 minutes. Remove from pan and cool completely on a wire rack.

In a small heavy saucepan, combine cream and 1 cup chocolate chips. Cook over medium heat, stirring constantly, just until chocolate melts. Cool 15 minutes. Drizzle chocolate glaze over the cake. Garnish with pecans. Let stand at least 1 hour before serving.

I found this in a search:
Adding a packet of dry instant pudding mix to cake batter increases the moisture of the cake itself after it has baked. You'll find several recipes at sites such as CakeMixDoctor.com that improve upon store bought cake mixes by adding instant pudding mixes.
On the Other: Different Ingredients
Cook and serve pudding mixes differ in ingredients from instant pudding mixes. Because cook and serve pudding mixes contain cornstarch as a thickening agent, substituting a cook and serve pudding mix for an instant pudding mix can affect the overall texture of your finished cake, making the cake denser.
Bottom Line
You should not substitute a cook and serve pudding mix for an instant pudding mix because cook and serve pudding mixes contain cornstarch which will adversely affect your finished cake.
Source:
The Food Timeline: Mixes
Recipe Zaar

I used to make the triple chocolate cake and always used sour cream for the heavy cream. Loved that cake.

Here's one I do now:

Chocolate sour cream cake
1 cup all-purpose flour
1 cup sugar
3 tablespoons baking cocoa
1 teaspoon baking soda
1/4 teaspoon salt
1 cup sour cream
1 egg
Instructions
1.Preheat oven to 350 degrees F. Coat an 8-inch square baking dish with cooking spray.
2.In a large bowl, combine all ingredients; beat until smooth. Pour batter into baking dish.
3.Bake 25 to 30 minutes, or until a wooden toothpick inserted in center comes out clean. Let cool, then cut into squares.

Top with whipped cream, frosting of choice, or dust with confectionate sugar.

Yummy with a scoop of good vanilla ice cream