Chill the whiskey in the refrigerator or freezer for about 15 minutes to ensure it's cold for blending.
Add the whiskey, ice cubes, and simple syrup into the blender in that order.
Secure the lid and blend on high until the mixture is smooth, thick, and evenly icy, about 30-45 seconds, you should hear the blender’s motor working steadily.
Check the texture—if it's too thick, add a splash of water or more whiskey and blend again briefly until it reaches a velvety, sherbert-like consistency.
Pour the frozen slushie into chilled glasses, allowing the icy, crackling texture to spill over the rims.
Optionally, garnish with a twist of lemon or a cherry for a colorful finish.
Serve immediately to enjoy the icy, smooth, and smoky flavors in every spoonful or sip.