CAKE: Preheat oven to 350. Combine cake mix & sugar. Add buttermilk and mix well. Add oil and mix well. Add eggs, one at a time, mixing well after each addition. Add vanilla and beat for 2 minutes with electric mixer.
Pour into 3 greased and floured round cake pans. Bake on 350 for approximately 18-20 minutes or until done. Cool in pans for 10 minutes then turn out onto a cloth and cool completely. Once completely cooled, cut each layer into half by using a bread knife or dental floss. Ice cake while icing is still warm.
*If you prefer you can spoon 4 to 5 tablespoons of batter into cake pans and bake approximately 8 minutes or until they start to brown around edges. Keep repeating until batter is used up. If I want those very thin layers I just make my Opel's Chocolate Stack Cake, which is posted.
ICING: Do not make icing until layers are cooled. In a medium-size heavy saucepan, combine sugar & cocoa. Add milk, stir and then add butter. Bring to a boil, stirring occasionally, then boil for an additional seven minutes. Turn off heat and add vanilla. Stir for one minute...icing will be thin but will harden later. Generously ice between cooled layers and top of cake. Icing will drip down sides, which will ice the sides. Keep spooning icing on top of cake as it puddles in bottom of cake plate. The icing will be thin then it'll get to the gooey stage while spooning on top but will harden later. Leave cake alone until icing hardens.
Note: I use a skewer or an ice pick to punch a few holes in cake as I'm icing it so that some of the icing can seep down into the layers.