In a medium sauce pan over medium high heat combine butter, water, and cocoa powder. Stir until mixture comes to a boil. Turn off heat and remove from burner.
In a large mixing bowl whisk together flour, white granulated sugar, baking soda, and salt. Pour in the hot butter/chocolate mixture. Stir to combine.
In separate medium mixing bowl whisk together eggs, milk, and vanilla extract. Pour mixture into batter and mix well.
Pour batter onto the prepared baking pan and smooth out evenly. Bake in preheated oven for 20 minutes until toothpick inserted in middle of cake comes out clean. Allow cake to cool for 15 minutes on stove top.