Sue, I have the Marlboro Chuckwagon cookbook, (circa 1970?) but there is no chocolate cake there. There is a Texas (Sourdough) Pecan Cake. However, I have a Wacky Cake that is similar to what you describe and mixes in the same pan it bakes in. Has been a standard of mine for years.

Wacky Cake with E-Z Penuche Icing

In ungreased 9x9 cake pan, mix:
1 cup sugar
1 1/2 cups flour
1/2 tsp. salt
3 Tbsp. cocoa
1 tsp. baking soda
Make a well in the dry ingredients and add:
1 Tbsp. vinegar
1/3 cup oil
1 tsp. vanilla
Fork stir to moisten. Add:
1 cup cold water and fork stir again.
Bake 350 degrees for 30 minutes.

I serve this with a quick penuche icing. It is very moist and makes a great "packable" dessert.

E-Z Penuche Icing

In sauce pan melt:
1/4 cup butter
Add:
1/2 cup brown sugar
Boil over low heat for 2 minutes, stirring constantly. Stir in:
2 Tbsp. milk
Cool to lukewarm. Gradually beat in:
1 cup sifted confectioner's sugar

Enjoy! Didn't know anyone knew of the old Marlboro cookbook.