Hi Deborah,
I have a feeling you are referring to tunnel of fudge cake. It originally was made from a mix and included a package of dry pudding mix that is no longer available.
This is the same cake made from scratch.

Tunnel Of Fudge Cake

CAKE
1 3/4 cups sugar
1 3/4 cups margarine or butter, softened
6 eggs
2 cups powdered sugar
2 1/4 cups Pillsbury BESTŪ All Purpose or
Unbleached Flour
3/4 cup unsweetened cocoa
2 cups chopped walnuts*

GLAZE
3/4 cup powdered sugar
1/4 cup unsweetened cocoa
1 1/2 to 2 tablespoons milk

Heat oven to 350 F. Grease and flour 12-cup Bundt or
10-inch tube pan. In large bowl, combine sugar and margarine; beat until light and fluffy. Add eggs, 1 at a time, beating well after each addition. Gradually add 2 cups powdered sugar; blend well. Lightly spoon flour into measuring cup; level off. By hand, stir in flour and remaining cake ingredients until well blended. Spoon batter into greased and flour pan; spread evenly. Bake at 350 F. for 58 to 62 minutes.** Cool upright in pan on wire rack 1 hour; invert onto serving plate. Cool completely.

In small bowl, blend 3/4 cup powdered sugar, 1/4 cup
cocoa and enough milk for desired drizzling consistency. Spoon over top of cake, allowing some to run down sides. Store tightly covered.

TIPS: * Nuts are essential for the success of this
recipe.

** Since this cake has a soft filling, an ordinary doneness test cannot be used. Accurate oven temperature and baking time are essential.

HIGH ALTITUDE - Above 300 feet:
Increase flour to 2 1/4 cups plus 3 tablespoons. Bake as directed above.