Add confectioners' sugar ½ cup at a time to the mixer running on medium-low speed. Allow the confectioners' sugar to fully incorporate before adding in another ½ cup. Repeat until no confectioners' sugar remains.
Deflate the buttercream by hand using a silicone spatula. Press the buttercream around the side of the mixer bowl until the frosting has smoothed and large air bubbles have been pushed out. This helps the buttercream pipe smoothly.
Match up macaron sides so they are paired with a similar size and shaped macaron. Pipe buttercream on one side of the macaron shell using your favorite piping tip. Gently place the matching side on top of the buttercream.