Pure soybean miso aged 2+ years, darkest and most intense.
Substitutes
Doenjang (Korean Soybean Paste) — 1:1 (both are strong, dark soybean pastes). Korean doenjang is the closest match to hatcho miso — both are 100% soybean fermented pastes (no grain added). Per 'Japanese Cooking: A Simple Art' (Tsuji), hatcho miso from Aichi prefecture is aged 2-3 years, creating an intense, dark, deeply savory paste. Doenjang undergoes similar long fermentation producing concentrated umami from glutamic acid. Both are saltier and more assertive than mixed misos. Excellent substitute in miso soup, sauces, and braises.
Red Miso — 1:1 (adjust salt to taste). Red miso is the closest substitute for hatcho, which is made from pure soybeans and fermented up to 3 years. Both develop deep umami through aspergillus oryzae fermentation producing 200-700mg/100g glutamic acid [McGee]. Red miso is slightly milder but similar intensity.
Miso — 1:1 (add 1/2 tsp soy sauce per tbsp for depth). Standard miso is lighter than hatcho. Add soy sauce to boost the dark, intense flavor.