In Star Trek: Deep Space Nine, the senior staff are often seen drinking Raktajino, whether in Quark's Bar, the Replimat, or Ops. We know that Raktajino is Klingon coffee... but what does that mean, really? I have my own headcanon, which draws from common fanon and speculations from Marc Okrand, the creator of the Klingon conlang.
A real Klingon raktajino is a cup of coffee made with robusta beans (native to Earth, grown on Q'onos), with a shot of ra'taj. Ra'taj is a Klingon herbal spirit or cordial, essentially grain alcohol distilled with a blend of spicy herbs native to Q'onos. The coffee was generally brewed grounds-in, in a saucepan, and decanted into cups, leaving some grounds in each cup. It was usually consumed black, though sometimes with targ milk and occasionally a mix of targ milk and targ blood. It is never served sweetened.
After the Khitomer accords, Klingon cuisine started filtering slowly into the Federation, often undergoing major changes to accommodate mainly human tastes. Federation raktajino is usually made with either Colombian arabica coffee, or a blend of arabica and robusta coffees (the latter being used by the station replicators on Deep Space Nine). Instead of ra'taj, it is made with an alcohol-free extract of the herbs used to flavor the spirit, allowing it to be consumed by Starfleet officers on duty. It is made with filtered or pressed coffee. It is almost always served with steamed milk and usually topped with frothed milk. It may be sweetened according to the tastes of the drinker, though if a replicator doesn't know your preference, it will be served unsweetened.
I haven't perfected a recipe, yet. My initial take is that the Klingon spices used are *vaguely* similar to cardamom, ginger, and black pepper, with a little cayenne pepper. The resulting Federation raktajino is then something like a pumpkin spice latte, something like masala chai, but not quite either.