Brew strong coffee, then chill it completely. If using espresso, brew 120ml espresso total, then mix it with 180ml cold water and chill briefly. The coffee must taste slightly stronger than you would normally drink it. Ice will dilute it, and weak coffee makes a flat, watery slushie. Because a substantial amount of ice is about to be blended directly into this base, the coffee needs to start with enough concentrated strength to survive that dilution and still taste like real coffee once it's fully icy and blended.

Stir the simple syrup and salt into the chilled coffee. Use 30ml syrup total for a balanced coffee-forward version or 40ml for a sweeter café-style slushie. Sweeten the coffee before blending. Sugar or honey added later does not dissolve evenly in a frozen slush and leaves grainy or syrupy pockets. Once ice is blended into the mixture, everything present becomes suspended within that icy, semi-frozen structure — a solid sweetener introduced at that stage has nowhere near the same opportunity to dissolve fully that it would in the still-liquid coffee, which is exactly why sweetening happens here, before any blending begins.

Add the sweetened chilled coffee, cold milk, and 250g ice to a blender. Blend in short pulses until the mixture becomes thick, icy, and pourable. Add the remaining 50g ice only if the texture is too loose. Stop while the drink still has a visible crushed-ice texture. Do not blend it completely smooth. A frozen coffee slushie should be icy and refreshing, not a creamy frappé shake. Pulsing in short bursts rather than running the blender continuously gives real control over how finely the ice breaks down — continuous blending would quickly push past the coarse, granita-like texture this drink depends on and into something closer to a smooth, uniform slush.

Check the consistency. If it is too thick to pour, add a small splash of cold coffee or milk. If it is too thin, add a few more ice cubes and pulse briefly. The finished texture should mound slightly in the glass but still be drinkable through a straw after a minute or two. This texture target — able to hold a slight mound while still remaining sippable — is a genuine middle ground worth checking for directly rather than assuming from a fixed blending time, since ice size and blender power both affect how quickly that consistency is reached.

Pour immediately into 2 glasses, about 250ml each. Serve right away. This drink has a short perfect window. It should look frozen, dark coffee-brown, and icy. If it sits too long, it separates into coffee liquid at the bottom and foam at the top. A freshly blended slushie's ice crystals begin melting and settling the moment blending stops, and that separation into liquid and foam is a genuinely fast process — serving immediately is what catches the drink at its intended, properly icy texture.