- Preheat the oven to 350-degrees (F). Prepare on 9x13 or two 9-inch cake pans by spraying with non-sticking cooking spray or coating with grease and then lightly sprinkling with flour. 
- In a large mixing bowl, combine the flour and sugar, then set it aside. 
- In a medium saucepan over medium heat, combine the butter, water and cocoa powder. Stir occasionally and cook until it comes to a boil. 
- While you're waiting for the cocoa powder mixture to boil, whisk together the milk, vanilla, baking soda and egg. Set aside.  
- Remove the boiling cocoa powder mixture from the stove and pour into the large mixing bowl with the flour and sugar. Stir together by hand or use a mixer to combine them thoroughly. (If you're using a mixer, be sure to start on low until the liquid has combined a bit before turning the mixer to medium.) 
- Once it is combined, pour in the egg mixture and mix until that is completely incorporated. 
- Pour the prepared batter (which will be relatively runny) into the prepared baking pan(s).  
- Bake in the oven for 35 to 30 minutes for 9-inch pans or 30 to 35 minutes for a 9x13-inch pan. You'll know the cake is finished when it starts to pull away from the edges of the pan.  
- Cool for 10 minutes before removing the cake from the pan. Cool completely before icing the cake.