Line two 9" round pans with parchment paper and spray with non-stick spray. Preheat oven to 350°.
Whisk together the flour, sugar, cocoa, baking powder, baking soda, and salt in a large mixing bowl.
Whisk in the milk, oil, eggs, and vanilla till smooth. Stir in the boiling water, mixing till smooth.
Divide batter evenly between the two pans. Bake at 350° for 30-35 minutes or till toothpick inserted in the center comes out clean. Cool for 10 minutes in the pans, then invert onto cooling racks.
Cool completely. Place one cake on a cake stand or serving tray. Spread about a cup of frosting on the top. Place the other cake on top. Frost sides and top of the cake.
For frosting, beat all ingredients in a bowl till smooth, adding enough milk to get a spreadable consistency.
Notes
Let the cake cool completely before frosting for the best results.