Assemble and frost according to our basic cake assembly directions. With the 8-inch round that you cut from parchment paper, create 8 equal wedges by folding the round in half 3 times. Lay the paper on top of the cake and use a toothpick to gently mark the cake edge at each fold of the paper. These marks will be your guides when making the swags.
Fill a pastry bag fitted with a coupler and a No. 2 plain tip with 1 cup melted chocolate. You may want to practice writing "happy birthday" on parchment paper first. Lightly mark 2 parallel lines on the cake with the edge of an offset spatula to use as guides. Hold piping bag 1/2 inch above cake and apply gentle pressure to let the chocolate run fluidly onto cake.
Make sure you have two colors of buttercream on hand -- light brown and dark brown. Place the light-brown buttercream and the dark-brown buttercream in separate piping bags, each fitted with a coupler. Fit the light-brown buttercream with the No. 100 ruffle tip. Starting with the tip at one of the toothpick marks, pipe a swag to the next mark. Continue piping swags all the way around cake.
Fit the dark-brown buttercream with a No. 13 star tip. Pipe a small rosette on top of the cake at the point where each swag meets the next. To make a rosette, hold the piping bag vertically above the cake and pipe a tight circle, drawing the tip up in a spiral toward the center as you release pressure. Refrigerate the cake for 30 minutes.
Switch the tip on the dark-brown buttercream to a No. 3 plain tip. Pipe 4 dots decreasing in size between the points of each of the swags. To make the dots vary in size, simply adjust the amount of pressure you put on the bag. Using a large offset spatula, transfer the cake to a cake stand or serving platter. Change the tip back to the No. 13 star tip.
Pipe the shell border around the bottom with dark-brown buttercream. Apply pressure to the bag while lifting it upward at 45 degrees. Decrease pressure; taper off, pulling bag down and right. Repeat, overlapping slightly, leaving small gaps beneath the columns of dots. With the light-brown buttercream and a No. 18 open-star tip, pipe larger rosettes in the gaps.