In a large bowl, combine cake mix, brownie mix, eggs, water, and vegetable oil, and whisk until batter is thick and smooth.
Line an 8x7-inch (20x17-cm) terra-cotta pot with foil, ensuring the sides are smooth as possible. Pour batter inside, leave 3 inches of space between the batter and the top of the pot.
Line a cupcake pan with green cupcake liners and fill each with about ¼ cup (32 g) of batter.
Put the terra-cotta pot and cupcake pan in the oven at the same time. Place a small pan or oven safe dish under the terra-cotta pot to catch any possible overflow of batter.
Bake cupcakes for 15 minutes. Bake cake for 45-60 minutes, or until a toothpick comes out clean.
In a medium bowl, combine 2 cups (455 g) buttercream frosting, 30 drops yellow food coloring, 10 drops red food coloring, and 4 drops brown food coloring to create a terra-cotta orange color.
In a separate bowl, combine 2 cups (455 g) of buttercream frosting and 1 drop of red food coloring to create a pale pink color.
Use the foil to lift the cake out of the terracotta pot, and carefully peel off the foil. Use an angled icing spatula to evenly ice the sides of the cake with the terra-cotta orange frosting. Use the spatula or icing scraper to remove excess icing and create a smooth finish. Add an inch (2cm) more of frosting to top rim of the cake, about 1-inch (2 cm) in thickness. Use the spatula or icing scraper to smooth the sides to create the rim of the pot.
Place a 2D piping tip inside a piping bag and fill it with the pale pink frosting. Twist the end of the piping bag and gently apply even pressure as you make a spiral design from the center out on each cupcake to create a rose.
Place two toothpicks in the center of the cake and use them to attach a cupcake to the center. Repeat around the rim of the cake with 4 more cupcakes spaced about 2 inches (5 cm) apart. Fold extra green cupcake liners into quarters and place one between each cupcake.