Measure out the bourbon, lemon juice, and simple syrup, then pour them into the blender.
Add the ice cubes to the blender, ensuring they fill up the container.
Secure the lid on the blender, then blend on high until the mixture is smooth and has a slushy, icy consistency—about 30-60 seconds. You should see a creamy, frosty texture with tiny ice crystals.
Pause the blender and check the texture; if it's too thick, add a splash of water or lemon juice and blend briefly to loosen it slightly.
Pour the slushie into chilled glasses, allowing the icy mixture to mound slightly above the rim.
Garnish with a lemon wedge or a sprig of mint if desired, and serve immediately for the best icy texture.