Fill a tall glass to the rim with pebble ice.

Pour the vanilla syrup over the ice and let it settle at the bottom before the soda goes in.

Tilt the glass and pour the orange soda slowly down the inside wall, stopping when the foam climbs near the rim and drops back.

Pour the heavy cream over the back of a spoon so it clouds the orange into pale, creamy streaks.

Stir once, notch the orange slice onto the rim, and serve right away.