Shake the lilac bunches to make sure there aren’t any small bugs hiding in there. Then delicately remove individual lilac blossoms from the stems.
Fill your jar with about 1/4 of the sugar. Then add a large handful of lilac blossoms. Repeat in layers with the sugar and lilac blossoms.
Seal the jar and shake it up. Store in a cool, dry, dark place for at least 2 days. Make sure you shake the jar at least once a day to help evenly distribute the lilacs amongst the sugar.
The sugar will get a little damp from the lilacs, so after a few hours or the next day, I like to dump the lilac sugar out on a parchment paper-lined tray or flat surface for an hour or so. That way the sugar dries up and loses the excess moisture. Return the lilac sugar to the jar and give it a shake.
After at least 2 days or (or up to a week), pluck or sift out out the flowers. You can use it in recipes (as this cake), in tea, or as a body scrub!