I've not tried this exact recipe, but had one similar and it was great! Unfortunately, the recipe for the Chocolate icing is missing. It was like a glaze icing--chocolate.. Maybe someone else can help. Enjoy!

* Exported from MasterCook *
Mexican Chocolate Cake
Recipe By : Houston Chronicle
Serving Size : 20 Preparation Time :0:30
Categories : Cakes & Pies Mexican
Amount Measure Ingredient -- Preparation Method
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1 stick margarine
1/2 cup vegetable oil
2 ounces unsweetened baking chocolate squares
1 cup water
2 cups all-purpose flour
1 teaspoon baking soda
2 cups sugar
1/2 cup buttermilk or sour milk (see note)
2 eggs -- beaten
1 teaspoon ground cinnamon
1 teaspoon vanilla extract
Mexican Chocolate Icing (separate recipe)
Preaheat oven to 350 degrees. Grease a 12-by-18-inch sheet-cake pan or a 13-by-9-inch baking pan. Combine margarine, oil, chocolate and water in a large saucepan over medium heat. Heat until chocolate is melted; set aside.
In a large mixing bowl, combine flour, soda, sugar, buttermilk, eggs, cinnamon and vanilla; add chocolate mixture to flour mixture and blend with a spoon (no need for an electric mixer). Pour batter into prepared pan and bake until cake tests done, 20 to 25 minutes in a sheet pan, longer in the baking pan. Spread warm icing on warm cake. Makes 15-20 servings.
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NOTES : For 1/2 cup sour milk, place 1 1/2 teaspoons vinegar in a 1-cup measure; add enough milk to equal 1/2 cup, then let stand 5 minutes.