Rub the inside of a chilled julep cup with one mint leaf, using the back of a spoon to gently bruise the leaf.
Combine remaining mint leaves, water, and sugar in a small bowl; mull gently until sugar dissolves. Add crushed ice, filling cup three-fourths full; strain water mixture over ice. Add additional ice to fill cup; add bourbon, and stir gently.
Chill at least 15 minutes. Before serving, garnish with fresh mint.