To toast the coconut, spread on a microwave safe plate. Microwave in 30 second increments at 40% power, stirring gently each time until the coconut is as brown as you'd like. Set aside to cool.
Preheat the oven to 350 degrees. Lightly spray the mini cupcake pans with cooking spray.
Make the brownie batter according to the directions on the package. Place equal amounts of brownie batter into thirty of the prepared cupcake cavities, about 1 tablespoon each.
Place the pans in the preheated oven and bake for 15-17 minutes. Remove the pan from the oven and place on a wire cooling rack. Let cool completely.
Place the buttercream frosting in the pastry bag and cut off a small hole at the tip. Squirt a small amount (about 1 tablespoon) of the buttercream in the center of each brownie (or spead the buttercream with a knife). Dip the brownie upside down in the plate of coconut, making sure plenty of it adheres to the frosting. Decorate by placing two to three colored jelly beans in the center of each nest.