Preheat oven to 350°F (175°C). Grease and flour two 9-inch round baking pans.
In a large bowl, stir together the flour, sugar, cocoa, baking powder, baking soda, and salt.
Add the milk, vegetable oil, vanilla extract, and eggs to the flour mixture and beat on medium speed until well combined.
Reduce speed and carefully add boiling water to the cake batter. Beat on high speed for about 1 minute to add air to the batter.
Pour the batter evenly into the prepared baking pans.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow to cool for about 10 minutes, then remove from the pan and cool completely.
For the frosting, beat the butter in a medium bowl until creamy. Gradually add the powdered sugar and cocoa powder, beating until smooth. Add the mint extract and milk, and beat until fluffy.
Frost the cooled cake layers with the mint chocolate frosting.