Begin by preparing your baking pan and preheating your oven to 350°F. In a medium saucepan, combine 1 cup of water, 4 tablespoons of cocoa, 1/2 cup of margarine, and 1/2 cup of shortening.
Bring this mixture to a boil over medium-high heat, stirring occasionally to guarantee the cocoa dissolves completely and the fats melt evenly.
This boiled base is vital to developing the cake's signature fudgy foundation and rich chocolate flavor. Once the mixture reaches a full boil, remove the saucepan from heat immediately.
Add the remaining ingredients to the hot mixture: 2 cups of flour, 2 cups of sugar, 2 eggs, 2 tablespoons of sour cream, 1 teaspoon of vanilla, 1/4 cup of milk, and 1 teaspoon of baking soda.
Stir everything together until just combined, being careful not to overmix the batter as this can result in a tougher cake. The batter should be smooth and uniform in color, with no streaks of flour remaining. The heat from the boiled mixture will help integrate all the ingredients thoroughly.
Pour the batter into a greased jellyroll pan, spreading it evenly to guarantee uniform baking. Bake at 350°F for 20 minutes, until a toothpick inserted near the center comes out with just a few moist crumbs—the cake should be set but still have that characteristic fudgy density.
The baking time may vary slightly depending on your oven, so begin checking around the 18-minute mark. Allow the cake to cool in the pan before serving, as this helps it set properly and maintain its structure.
