Preheat the oven to 350˚F. Line a muffin pan with paper liners and set aside.
In a medium bowl, mix together the flour, cocoa powder, baking soda and salt. Set aside.
Cream together the sugar and the egg. Add in the vanilla, milk and butter until fully combined. Slowly add in the dry ingredients to the wet ingredients until fully incorporated.
Divide the batter evenly into the prepared muffin pan, and bake in the preheated oven for 15-18 minutes, or until a toothpick inserted comes out clean.
To make the frosting: mix together all of the frosting ingredients in a large bowl. Add in more or less milk, depending on your desired consistency.
Once the cupcakes are cool, place the frosting in a ziplock bag with a corner clipped, or a piping bag. Swirl the frosting on each individual cupcake.