Please suggest a substitute for espresso powder which is not made from coffee

You can pretty much substitute coffee with roasted grain coffee-like powder of any brand you like. It works as a drop-in replacement in drinks and foods really well. After all, coffee is just a roasted grain, too.

If you need instant coffee, one that dissolve in water or baked goods, I know for a fact that Inka brand makes instant roasted grain "coffee" that can be used as substitute. I don't know about other brands quality, but you can try ones found on Amazon.

I personally prefer ones with sugar beet and chicory, some brands also use dandelion. Additions are there to emulate coffee better, to make taste more rich and complicated. Basically, if it tastes good and is bitterish enough, it'll work.

I'm not affiliated with linked sellers. It is just an example.

If it's primarily a chocolate recipe the coffee is there to add flavour. You may be able to simply omit it. I would generally replace it with the same quantitity of (unsweetened) cocoa powder as otherwise the recipe may be a little too sweet.

If you find the overall recipe lacking in flavour after that, try more cocoa or adding a tiny bit of a compatible flavour, for a different but still good effect. One thing that works well with chocolate is vanilla, another is ginger (though too much and it will dominate - good, but not what you asked for). If the espresso powder is dissolved in liquid before adding, you can use liquid extracts to replace some of the liquid (but be careful what you buy if you also avoid alcohol as many who avoid coffee do). This gives you slightly more choice, as you can use a little almond extract, or even mint, or replace the liquid with orange juice. Of the flavours I mentioned, the almond and the vanilla would be best for just adding a subtle bit more flavour, while the others are conventionally used with chocolate in some recipes and ould go well, though change the end result.

If the recipe is a mocha recipe, or there's a lot of coffee in there, I suggest looking for a similar recipe with no or less coffee. This is always an option.

It won't be coffee flavored (maybe that's not a bad thing), but what about ground black cardamom? I have yet to try it (I am ordering it shortly), but Alton Brown has updated some of his old recipes. His updated chocolate syrup uses black cardamom to enhance the coffee syrup. From his description in the episode, it sounds like more of an herbal compliment, unlike espresso powder's enhancement of the bitter flavors.

See new recipe here.

I fully plan on playing around with black cardamom and chocolate.

Chaga mushroom powder has a similar flavor profile to a lot of coffees and is also super good for you. Coffee is a roasted cherry pit, not a grain, so I'd avoid those. There are a lot of acidic and tannic qualities in brewed coffee, so even a black tea powder may work well! Like another poster said, it's really just to add depth of flavor to the chocolate.

Barley Cup was the go-to instant coffee substitute back in the day - roasted cereal and chicory in an easy-to-use powder.

Artificial coffee flavour seems like the right solution for your problem.