Preheat the oven to 350˚F. Line a muffin pan with paper liners and set aside.
In a medium bowl, mix together the flour, 1/4 cup 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: Cream together the butter and cream cheese on high until light and fluffy. Reduce the speed, and add in the icing sugar and vanilla. Mix until fully incorporated. Place in a ziplock bag or a piping bag, and pipe the frosting onto the cupcakes.
Decorate the cupcakes with the black icing gel and m&ms to make it look like Christmas lights.