I can remember that recipe & I am sure I have it in my clippings collection. It was a Hellmans mayonnaise recipe. Anyway there are a lot of versions in the internet because as you said it is an excellent recipe. I did it frequently before my sugar problems.

Chocolate Mayonnaise Cake

2 cups unsifted all purpose flour
1 cup sugar
1 cup unsweetened cocoa
1 teaspoons baking powder
1 teaspoon baking soda
1 cup mayonnaise
1 cup cold coffee
1 teaspoon vanilla

Grease 9x9x2 inch baking pan;* Line bottom with waxed paper. Grease waxed paper. Mix and sift flour, sugar, cocoa, baking powder and baking soda into large mixing bowl. Stir in mayonnaise. Gradually stir in coffee and vanilla until smooth and blended. Pour into prepared pan.*(A heart-shaped pan of approximately the same volume is a nice touch.) BAKE AT 350 degrees for 40 to 45 minutes or…until cake tester inserted in center comes out clean. Cool completely. Remove from pan. Remove waxed paper. FROST as desired.

Alternate topping:
Melt 8 oz. chocolate chips and spread melted chips while warm over cooled cake.