A note on butter: For these whipped shortbread cookies, we prefer to leave the butter out overnight (although the warmer your kitchen, the less time it will need to come up to temperature). To ensure your batter becomes soft enough to be easily piped, the butter should be so malleable that it can be dipped into with a spoon.
*Cooler butter will still blend into the dough, but it will make piping the rosettes much more difficult. Should this happen, let the dough sit and warm further before continuing with piping.
Kitchen Tip: Trace 1 1/2-in. circles onto parchment. Flip the parchment paper over and use circles as a guide when piping rosettes.