Preheat oven to 350ºF. Prepare two 9-inch cake pans greasing with butter and then flouring. Add flour, sugar, cocoa, baking powder, baking soda and salt in a large bowl. Whisk through to combine until combined well. Add milk, vegetable oil, eggs, and vanilla to flour mixture and mix together on medium speed of a mixer until well combined. Reduce speed and carefully add boiling water to the cake batter. Beat on high speed for about 1 minute Distribute cake batter evenly between the two prepared cake pans. Bake for 30-35 minutes, until a toothpick oin the center comes out clean. Remove from the oven and allow to cool for 10 minutes.

For the Chocolate Frosting:
Add cocoa to a large bowl or bowl of stand mixer. Whisk through to remove any lumps. Add butter with cocoa powder. Whisk together until well-combined. Add sugar and milk to cocoa mixture by adding 1 cup of sugar followed by about a tablespoon of milk. After each addition has been combined, turn mixer onto a high speed for about a minute. Repeat until all sugar and milk have been added. Add vanilla extract. If frosting appears too dry, add more milk, if it appears too wet add more icing sugar.