Preheat oven to 350°F. Fill muffin tin with paper liners.
In a large bowl of an electric mixer add all the dry ingredients and stir to combine.
In a large measuring cup combine the wet ingredients and stir.
With the electric mixer set on low add the wet ingredients and continue mixing for about 2 minutes until smooth and shiny.
If using add-ins fold them in with a spatula making sure they are evenly mixed in the batter.
Fill the cupcake liners about ⅔ full. Using a metal ice cream scoop with a thumb grip is a good way to enure cupcakes are all the same size and will bake evenly.
Bake for 18 minutes or until a toothpick comes out clean. Turn the pans around in the oven half way through the cooking time to ensure the cupcakes cook evenly.
Place the pan of cupcakes on a cooling rack and allow to sit for 2 minutes. Gently remove the cupcakes from the pan and place on the cooking rack to cool completely.
Spread with chocolate icing.
CHOCOLATE ICING
Combine butter, icing sugar and cocoa in a food processor with a metal blade. Blend for 30 seconds.
Add vanilla and milk and blend until smooth.
Using a spatula fill a piping bag with icing. Using even pressure squeeze the piping bag and draw in circles on the top of each muffin starting from the outside edge and ending in the enter.
Alternately you can spread the icing on each cupcake with a smooth spreader.