Pour the chilled champagne into the blender jar.
Add the ice cubes to the blender, distributing evenly over the liquid.
Drizzle in the simple syrup for sweetness, adjusting based on your preference.
Pulse the blender a few times to start crushing the ice and mixing the ingredients gently.
Blend on high until the mixture is smooth, icy, and slightly slushy, about 30-45 seconds.
Turn off the blender and check the texture—the drink should be bubbly, cold, and fluffy.
Scoop the champagne slushie into chilled glasses and garnish with a twist of lemon or a small sprig of mint if desired.