Japanese iced coffee — also called flash brewing — is filter coffee brewed hot directly onto ice, so it chills the instant it is made.
Why it beats cold brew for some coffees
Cold brew never gets hot, so many of the aromatic and acidic compounds never extract. The result is smooth and sweet and loses the brightness that makes a light Ethiopian interesting.
Flash brewing extracts hot — so everything comes out — and then chills instantly, trapping the volatile aromatics that would otherwise evaporate as the coffee cooled slowly.
The result is intensely aromatic and bright, and it takes four minutes rather than sixteen hours.
The method
Work out your total water — say 300 g. Split it: about 40 per cent as ice in the server (120 g) and 60 per cent as brew water (180 g).
Use your normal coffee dose for the total water: 18 g for 300 g.
Brew as a normal V60 with the 180 g of hot water, straight onto the ice. Grind one step finer than usual, since you are using less water.
Swirl when it finishes, and serve.
Which coffee
Bright, fruity, light roasts. Washed Ethiopians and Kenyans are exceptional this way.