Preheat oven to 350. Lightly grease and flour two 9 inch cake pans.
Put all cake ingredients in large mixing bowl. Mix as directed on box.
Pour into pans. Bake 23 to 28 minutes, or until tests done.
Remove from oven and let cool for 10 minutes. Run a sharp knife around edges of cakes, then turn cakes out onto a plate, then invert on a wire rack (so the cakes are right side up) to finish cooling, at least 30 minutes more.
While cake is cooling, make frosting:.
Place cocoa into large mixing bowl and pour the boiling water over it. Stir with a wooden spoon until cocoa comes together in a mass.
Add butter and beat on low speed of mixer for about 30 seconds, or until well combined.
Add powdered sugar and vanilla, then beat on low for about 1 minute, then increase speed to medium until the frosting lightens and is fluffy, about 2 minutes more.
Add additional boiling water, 1 tsp at a time, or powdered sugar, 1 Tbs at a time, until frosting is of desired consistency.
Use frosting to frost the cooled cake layers.