6 small unglazed terra cotta pots, about 2 1/2 inches tall with a 2 1/2-inch diameter; small piping bag fitted with a small closed-star tip
Preheat the oven to 350 degrees F. Line a baking sheets and an 8-by-8-inch baking pan with parchment paper.
Prepare the brownie batter according to the package instructions.
Cut 6 small circles out of a third sheet of parchment paper to line the bottoms of 6 small unglazed terra cotta pots and block off the drainage hole.
Set the pots on the prepared baking sheet and fill them about half full with the brownie batter. Pour the remaining brownie batter into the prepared baking pan. Bake until a wooden pick inserted into the center comes out clean, 20 to 22 minutes for the brownie cupcakes and about 35 minutes for the pan of brownies. Cool both completely.
Remove the brownies from the baking pan and crumble into a large bowl. Add the chocolate buttercream and mix and smoosh it until it is well mixed and the crumbs hold together well. Form the brownie mixture into 6 cactus shapes, some spherical and some cylindrical. Make sure the shapes are smooth and well compacted. Stand them on the baking sheet used for baking and refrigerate until just firm, about 20 minutes.
Meanwhile, microwave the candy melts and coconut oil in a small heatproof bowl for about 1 minute, stirring every 20 seconds, until smooth and a nice coating consistency.
Pour the melted candy melts over the top of each cactus until completely coated. Sprinkle each lightly with sprinkles on all sides.
Put the pink icing into a piping bag with a small closed-star tip and pipe small "flowers" onto the tops of each cactus. Spread the remaining icing on the tops of the cupcakes. Sprinkle the graham cracker crumbs onto the frosting all around the edge (not in the center).
Trim any pooled candy coating from the bottom of each cactus with a sharp paring knife, and then place a cactus onto the center of each cupcake. Serve immediately or refrigerate until ready to serve (let the cupcakes sit at room temperature 1 hour before serving).