Place mint leaves, 1-tablespoon sugar and a small measure of bourbon in a crystal tumbler.   Gently muddle with a spoon.  Place a scoop of freshly cracked ice in tumbler and stir vigorously.  Top off with more ice.  Add equal measures of bourbon and sparkling water to fill glass.  Place a sprig of fresh mint on top; zest with a twist of lemon and dust with powdered sugar.