Prepare the Oven: Preheat the oven to 325°F (160°C), or 140°C if using a fan oven. Line a 12-cup muffin pan with cupcake liners.
Combine the Wet Ingredients: In a medium mixing bowl, whisk together the eggs, granulated sugar, vegetable oil, and vanilla extract until combined.
Add the Milk: Stir the milk into the egg mixture until incorporated.
Add the Dry Ingredients: Add the flour, cocoa powder, baking powder, and salt to the same bowl. Mix just until the ingredients are combined and no dry streaks of flour remain. Avoid overmixing; the batter will be fairly thin and runny.
Fill the Liners: Divide the batter evenly among the 12 cupcake liners, filling each approximately three-quarters full.
Bake the Cupcakes: Bake for 18–20 minutes, or until a toothpick inserted into a cupcake comes out clean.
Cool Completely: Allow the cupcakes to cool completely before adding the frosting so the buttercream does not melt during decorating.
Whip the Butter: Place the softened butter in a mixing bowl and beat for 2 minutes, until it becomes light and creamy.
Add the Powdered Sugar: Gradually mix in the powdered sugar. Once incorporated, increase the mixer speed and whip for 1 additional minute.
Finish the Frosting: Add the vanilla extract, milk, and green food coloring. Mix again until the frosting is smooth and evenly colored. The source specifies adding about ½ teaspoon of food coloring initially; add more if a brighter green shade is desired.
Create Fondant Stars: If using the optional star decorations, roll out the gold fondant, brush it with gold lustre powder, and cut out small star shapes using a star punch.
Make Fondant Packages: If desired, shape the red fondant into small cubes. Decorate the pieces with white frosting to create little gift packages and add frosting bows on top.
Pipe the Christmas Trees: Pipe a generous, tree-shaped swirl of green frosting onto each completely cooled cupcake, using a large open star tip. Decorate the frosting trees with sprinkles or sugar pearls.
Add the Finishing Decorations: Place the optional gold fondant stars on top of the frosting trees and add the small red fondant packages around the cupcakes as desired.
