In a medium bowl, combine lemon zest and sugar. Using fingers, rub together until sugar feels damp and mixture is fragrant, about 1 minute. Let stand for 20 minutes, rubbing sugar mixture together every few minutes until very fragrant.
In a blender, combine lemon juice, sweetened condensed milk, sugar mixture, and 1 cup ice. Blend until smooth and creamy, about 30 seconds.
Add remaining 3 cups ice and blend until thick and fluffy, about 30 seconds. Divide evenly between 2 chilled glasses. Top with additional lemon zest, if desired, and serve immediately.
