Pour the freshly brewed coffee into a measuring cup and let it sit until it reaches room temperature, ensuring it isn't hot when blending.
Add the cooled coffee into the blender along with the ice cubes and sweetener.
Blend everything on high speed until the mixture is smooth, frosty, and with a thick, slushy consistency—about 30 to 40 seconds.
Pause to check the texture; if it's too thick, add a splash more coffee or water and blend briefly again.
Pour the coffee slushie into chilled glasses, watching the icy mixture mound slightly over the rim.
Serve immediately with a straw or a stirring stick to enjoy the crisp, icy coffee treat.