OK. First off, these coffees are meant to be served at an 8:1 to 12:1 water to coffee brew ratio; think Mochapot or Aeropress, although when I was growing up (Munich, same coffee culture as Vienna), it was filters. The preferred additive is "Schlag," whipped cream or heavy cream.
The classic version is Mocha-Java, which can be any Yemen or Ethiopian in a medium-light and any Indonesian in a medium dark roast. An alternative for the darker roast are Kenyan SL28s or Rwandan bourbons. Gilly's in New York sold a Colombian medium and Kenyan dark version to the mid town hotels; maybe they still do. Guatemalan bourbons, especially Antiguas, can also work for the darker roast, although you'll need on with a heavy body. You are looking for a distinguished roast taste, spicy, caramelly, and complex. If you're into whiskeys and exotic oak aging of all kinds, you'll know what I mean. This means the ideal top end coffee should be mildly fruited, but not very acidic (think of the mash in a whiskey).
There's a small difference between this style and the Seattle espresso blends from the aughts: the espresso had to have a lot of naturals, usually Brazils, to make for the thick crema. I'm guessing a blend of medium light Brazil and medium dark semi-washed Sumatra could go both ways. Black Cat used to be a Seattle style blend, but it's third wave blend now.
The proportions that will work best depend on the power of the lower end. Sumatras are very muscular, and probably only need 25% to 40%. The others are up at 50%. Notice these are always post roast blends and plan your roast batches accordingly. If you single dose, you can mix the coffees for each serving until you have the proportions down.
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