Pour the cooled brewed coffee into the blender, adding the simple syrup for sweetness.
Add the ice cubes to the blender, creating a generous mound on top of the liquid.
Secure the lid on the blender and blend on high until the mixture is smooth and frosty, with a slushy, ice-crystal texture.
If desired, add milk or a milk alternative to the blender, then pulse a few times to incorporate evenly.
Stop the blender and check the consistency—should be thick, creamy, and icy to the scoop.
Pour the slushie into two chilled glasses, aiming for a frosty, slightly piled presentation.
Garnish with a few coffee beans or a swirl of whipped cream if desired, and serve immediately for the best icy texture.