Preheat oven to 350 degrees F (175 degrees C). Butter a 9x13-inch baking dish.
Whisk sugar, margarine, and eggs together in a large bowl until smooth and creamy. Add flour, buttermilk, cocoa, and baking soda and mix. Stir boiling water into mixture just until batter is mixed. Pour batter into the prepared baking dish.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, 30 to 35 minutes. Sprinkle chocolate chips over cake and let sit until chocolate begins to melt; spread over cake, creating an instant chocolate frosting.