Add the vanilla ice cream, milk, sugar, and citrus zest to the blender. Blend on high until the mixture is smooth and creamy, with no lumps remaining.
Pour in the sparkling water and blend again briefly just until combined, about 10 seconds. The mixture will start to foam and lighten in texture.
Stop the blender and check the consistency; it should be thick but scoopable and sparkling with tiny ice crystals visible on the surface.
Pour the mixture into glasses, filling them to the brim. The icy slush will have a frosty, snow-capped appearance with a slight shimmer from the sparkling water.
Serve immediately with colorful straws or garnish with a twist of citrus for added flair. The drink should be cold, frosty, and irresistibly refreshing!