Brew a strong cup of coffee and let it cool to room temperature, ensuring it's not steaming hot when added to the blender.
Pour the cooled coffee into the blender, then add the milk and sugar. Blend briefly until the sugar dissolves and the mixture is combined smoothly.
Add the ice cubes to the blender, then start blending on high speed. Continue until the ice is finely crushed and the mixture reaches a thick, slushy consistency.
Check the texture; it should be cold, smooth, and scoopable, resembling a soft-serve consistency. If it's too thick, add a splash more milk and blend briefly again.
Pour the coffee slushie into chilled glasses, and if desired, garnish with a sprinkle of cocoa powder or a coffee bean for presentation.
Serve immediately with a straw or spoon, enjoying the icy, caffeinated refreshment in every sip.