This is the one I've used a lot, it is in a cookbook titled Hershey's Fabulous Desserts

HERSHEY'S CHOCOLATE FUDGE CAKE

3/4 cup butter or margarine
1 2/3 cups sugar
3 eggs
1 1/2 tsps. vanilla
2 cups all purpose flour
2/3 cup Hershey's unsweetened cocoa powder
1 1/4 tsp. soda
1/2 tsp. baking powder
1/2 tsp. salt
1 1/3 cups milk

Preheat oven to 350 degrees F. Grease and flour two (9-inch) round pans or one (9x13x2-inch) pan.

In large mixer bowl cream butter, gradually add sugar and continue beating until light and fluffy. Add eggs, one at a time, beating well after each addition. Blend in vanilla. Beat at medium speed 5 minutes, scraping sides of bowl occasionally.

Combine flour, cocoa powder, baking soda, baking powder and salt; add to creamed mixture alternately with milk, beginning and ending with dry ingredients and beating just enough to blend. Pour batter into prepared pan(s).

Bake 35-40 minutes or until tester comes out clean. Cool round cakes 10 minutes on wire rack,remove from pans, cool completely. Rectangular cake may be left in pan if desired. Frost with Chocolate fudge Frosting.

CHOCOLATE FUDGE FROSTING

3/4 cup butter or margarine
1 cup Hershey's unsweetened cocoa powder
4 cups powdered sugar
1/2 cup hot milk
2 tsps. vanilla

In small saucepan over low heat, melt butter; add cocoa, stirring constantly until smooth and slightly thickened. Remove from heat, set aside to cool slightly.

In large mixer bowl combine sugar and milk, beat until smooth. Add chocolate mixture and vanilla. Beat on medium speed until smooth and slightly thickened, 5 to 10 minutes. Cool at room temperature to spreading consistency, about 1/2 hour.

Makes frosting for one (2-layer) cake
For 13x9x2 cake, halve the ingredients.