Preheat your oven to 350°F (175°C) and line a cupcake pan with liners.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
Add the milk, vegetable oil, and vanilla extract to the dry ingredients and mix until well combined.
Carefully add the boiling water to the batter and mix until smooth. The batter will be thin.
Pour the batter into the cupcake liners, filling each about ⅔ full.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean. Let the cupcakes cool in the pan for 10 minutes, then transfer to a cooling rack to cool completely.
For the frosting, beat the softened butter until creamy. Gradually add the powdered sugar, beating until smooth. Add the vanilla extract and milk, and beat until fluffy.
Frost the cooled cupcakes with the vanilla frosting.