Preheat the oven to 350 F. Line a muffin pan with cupcake liners.

In a large bowl, combine the sugar, flour, baking soda, baking powder, salt, and cocoa powder.

Add the egg, milk, oil, and vanilla extract and beat on medium speed until fully combined, about 2 minutes.

Stir in boiling water (batter will be thin and runny).

Pour batter into prepared cupcake liners and bake for 15 minutes or until a toothpick inserted into the center of the cupcake comes out clean.

Cool cupcakes in the pan for 10 minutes before transferring to a wire rack to cool completely.

On medium speed, beat butter until light and fluffy, about 2 minutes.

On low speed, beat in the marshmallow fluff until just combined, about 30 seconds.

Gradually beat in sugar and vanilla, then beat on medium speed until fluffy.

Scoop a small amount of the frosting into a separate bowl. Add a few drops of green gel food coloring and stir until well combined.

Add a few drops of orange gel food coloring to the large bowl and stir until well combined.

Transfer the green icing to a piping bag fitted with the leaf (Wilton 352) piping tip.

Transfer the orange frosting to a piping bag fitted with a large round (Wilton 1A) piping tip.

Pipe a small mound of orange frosting in the center of the cupcake.

Pipe frosting lines from the bottom of the frosting mound to the top of the frosting mound to make the pumpkin sides.

Place a piece of the cut up Twix bar into the center of the frosting, where all the ends meet, to make the pumpkin stem.

Using the green icing, pipe a small leaf next to the stem.

Gently press two chocolate rounds onto the sides of the cupcake to make Mickey ears.