Using a small pastry brush, brush 1 rosemary branch with egg white (do not dip branches into egg white). Working over a wire rack and wax paper, sprinkle rosemary heavily with sugar through a wire-mesh sieve, turning to coat. Shake off excess sugar; place rosemary on wire rack to dry. Repeat with remaining rosemary, egg white, and sugar. Let rosemary stand at least 8 hours or until dry; use immediately, or keep up to 1 week, uncovered, at room temperature.
Tip: Make your own superfine sugar by pulsing granulated sugar in a food processor for about 30 seconds.