Heat oven to 350 degrees. Stir together sugar, flour, cocoa, baking powder, baking soda and salt in a large bowl. Add eggs, milk, oil and vanilla. Mix until well blended. Lastly, stir in boiling water and mix well. Mixture will be thin compared to boxed cake batter. Prep desired cake pan with cooking spray (esp. if you use round cake pans). Pour batter into pan and cook for 30 minutes or until toothpick comes out clean. For Frosting, melt butter and stir into powdered sugar, cocoa, milk and vanilla. If you want, you can slowly add in milk to get to desired consistency.