a pastry bag fitted with a large open star tip (no. 199); a pastry bag fitted with a large closed star tip (no. 846); a pastry bag fitted with a small star tip (no. 15); a pastry bag fitted with a small round tip (no. 2)
For the cupcakes: Preheat the oven to 350 degrees F. Line 18 cups of two 12-cup muffin pans with paper liners.
Sift together the flour, baking powder and salt in a medium bowl; set aside.
In the bowl of stand mixer, cream the granulated sugar and butter until fluffy, about 3 minutes. Add the eggs one at a time, beating well after each addition and scraping down the bowl as needed. Beat in the vanilla.
Reduce the mixer speed to medium low. Beat in half of the flour mixture, then all of the milk, then the remaining flour mixture and finally the champagne. Mix until just combined. Using an ice cream scoop, fill each muffin cup two-thirds full with batter.
Bake until the tops of the cupcakes spring back when gently pressed, 18 to 20 minutes. Transfer the pan to a rack and let cool 5 minutes, then remove the cupcakes to the rack to cool completely.
For the buttercream frosting: Combine the confectioners' sugar, butter and salt in the bowl of a stand mixer fitted with a paddle attachment. Mix on low speed until mostly incorporated. Add the vanilla, increase the speed to medium high and mix until smooth. Adjust the consistency with milk as desired.
Put 1/2 cup of the frosting in a one small bowl and another 1/2 cup in a second small bowl. Tint the first bowl a light shade of green and the second bowl a darker shade of green. Add 1/4 cup of plain frosting to a third small bowl and tint it pink. Add 1/4 cup of plain frosting to a fourth small bowl and leave it plain. Add enough of the green food coloring to the remaining plain frosting in the mixer bowl to tint it dark green (darker than the other 2 bowls of green frosting).
For decorating: Frost the top of 12 of the cupcakes with an even layer of the darkest green frosting. Place the graham cracker crumbs in a small bowl and dip the edges of the cupcake in the crumbs, rotating to coat the edges.
Make aloe plants by forming 12 teardrop-shaped pieces of green gum paste. Using scissors, snip the pointed end of each teardrop halfway to the bottom in one direction, then across in the opposite direction. Spread the tips gently and place one aloe plant onto each of the frosted cupcakes.
Fill a pastry bag fitted with a large open star tip (no. 199) with one of the lighter shades of green frosting. Fit a second pastry bag fitted with a large closed star tip (no. 846) with the other lighter green frosting. Fill a pastry bag fitted with a small star tip (no. 15) with the pink frosting. Fill a pastry bag fitted with a small round tip (no. 2) with the white frosting.
Pipe cacti onto the cupcakes using the 2 shades of green frosting. Pipe cactus flowers with the pink frosting and finish by piping small white dots on the cacti for the needles.