We are going to add a clamshell border to our swirl design cake here. I'm using a chocolate Swiss meringue with a smaller star tip. This one is actually number 32. It's going to give us a nice ridged pattern as we go around.
The trick is to let the butter cream build up, and then to pull away. I'm going to start at the edge here. I'm going at about a 35-40 degree angle, and I just squeeze, squeeze, squeeze and then pull away. Squeeze, squeeze, squeeze, and you can see they start to overlap each other.
You see, I'm moving the turntable more than I'm moving the bag. That way, it's always at the same angle, so they'll stay as consistent as possible.
I'm doing this border on the bottom of the cake to really tie it together so it has sort of a floral theme, but you can do this on the top of the cake, around the middle. It's just a really fun border. You can never get to them to end perfectly. Your best bet is to do a little circle bead and let it trail off at the end.
That's how, using butter cream, you can make a clamshell edge.