Pour the chilled champagne into a measuring cup to make it easier to blend later.
Add the simple syrup and fresh lemon juice to the champagne, stirring gently to combine and balance the sweetness with a tangy note.
Transfer the mixture to a freezer-safe container and freeze for about 2 hours, until it is thoroughly chilled and slightly slushy.
Place the ice cubes in a blender and add the frozen champagne mixture on top.
Pulse the blender until all ice is broken down and the mixture becomes smooth, velvety, and icy in texture.
Pour the slushie into chilled glasses, allowing the fizzy bubbles to shimmer through the icy blend.
Garnish with a lemon twist or a sprig of fresh mint if desired and serve immediately for the freshest, frosty experience.