Preheat the oven to 375 degrees. Line 24 miniature muffin tins and 4 large muffin tins with cupcake liners. Set aside.
Sift the flour and sugar together. Set aside.
Bring to a boil the oil, butter, cocoa powder, and water, stirring until smooth and combined.
Add butter and cocoa mixture to the flour and sugar and stir until incorporated. Add buttermilk, baking soda, salt, vanilla, and the egg. Mix until fully combined.
Using a tablespoon-sized cookie scoop, portion the batter between 24 miniature cupcake liners and 4 large cupcake liners filling ⅔ full. Bake mini cupcakes for 12 minutes and the large cupcakes for 18 minutes.
In the bowl of a stand mixer fitted with a paddle attachment, add the butter and mix until smooth and creamy.
Add the powdered sugar and mix until well combined and light and fluffy.
Add the vanilla extract and milk. Mix until combined. Add more milk if needed to reach the desired spreading consistency.
Remove ½ cup of frosting to a small bowl. Add 1 drop of brown food coloring gel and stir to incorporate. Adjust the color as desired. Set aside.
To the remaining frosting, add 4-6 drops of green food coloring gel and mix to combine. Adjust color as desired.
To frost the cupcakes, add the brown frosting to a piping bag fitted with a piping tip (I used a 4B tip). Frost two of the large cupcakes with the brown icing.
Add the green frosting to a piping bag fitted with a piping tip (I used a 2D tip). Frost the miniature cupcakes with the green frosting.
On a large platter, tray, or board, arrange 21 of the miniature cupcakes in the shape of a Christmas tree. Use 2 large cupcakes to form the trunk of the tree. Adorn some of the green cupcakes with the Christmas light cake decorations. Dust with powdered sugar. Serve.
Any leftover frosting can be used the decorate the remaining cupcakes. Enjoy these as a snack!