To Make the Cake:

1. Preheat the oven to 150 degrees Celsius (about 300 degrees Fahrenheit) and place your cupcake liners in the cups of the tray.
2. Combine the flour, sugar, cocoa powder, and baking soda in a mixing bowl.
3. In a separate bowl, whisk together the eggs, vanilla extract, melted butter, oil, milk, and vinegar.
4. Slowly add the wet mixture into the dry mixture and combine until the the batter is smooth with no lumps. Next, fold in the chocolate chips or other mix-ins, such as nuts, dried fruit, or sprinkles.
5. Use a metal spoon to equally divide the mixture into the liners, leaving each liner about 3/4 full. Bake the cupcakes for 15-18 minutes, or until golden and springy to the touch. Set the tray aside until the cupcakes are completely cool.

To Make the Buttercream Icing:

1. Using a whisk or hand mixer, cream together the powdered sugar and butter until it reaches a light and creamy consistency. Fold in the warm melted chocolate and your desired amount of food coloring, if you're using it.
2. If you have a piping bag at hand, pipe the icing on top of the cakes for decoration. You can also decorate your cupcakes using a spoon, spatula, or butter knife.

To Prepare the Royal Icing:

1. If you're decorating your cupcakes with royal icing, roll the icing out into thin sheets and use the rim of a cup to cut the icing into circular disks. Lay the discs over each cupcake and shape them around the tops of the cupcake gently using your hand or the back of a spoon.