For the frosting, beat the unsalted butter until smooth.
Alternate adding the powdered sugar (slowly), vanilla, and milk. Mixing until well combined. If needed, add additional powdered sugar if the frosting is too thin, or a splash of milk if it is too thick.
Take about 1 cup of the frosting and set aside.
To the larger bowl of frosting, add a generous amount of orange gel food coloring, and mix until well combined.
Add a generous amount of green gel frosting to the cup of frosting you set aside, and mix well to combined.
To a piping bag, add a large round decorating tip. Add the orange frosting to this bag.
To another piping bag, add a medium writing tip. Add the green frosting to this bag.
Start piping the pumpkins onto the cakes by starting from the outside and adding your pumpkin lines until you get to the final middle line.
Once you have pumpkins, use the green frosting to pipe vines onto them.