Substituting maple syrup for maple extract

One tablespoon of apple brandy in a recipe of that volume is an accent; it may be nice to have, but it is not going to dramatically transform the outcome.

The obvious choice is to simply omit it.

Otherwise, some reasonable substitutions include:

• Vodka. While it doesn't bring a specific flavor, it will provide alcohol to help dissolve those flavor components that otherwise would not be as apparent, perhaps intensifying the flavor of the cake.
• Bourbon. Compliments the vanilla, and provides a counterpoint to the apples.
• Brandy, either grape or another fruit like pear. Another, similar flavor accent.

There is little point in reducing soft cider because it will not contain alcohol, and the quantity is not enough to matter, unless (and then only maybe) you make a syrup--in that case, however, you would be more substantially transforming the recipe.

Hard cider has some alcohol, so might work (after all, Calvados or apple brandy is distilled hard cider), but you don't want to reduce it, which would drive a significant percentage of the alcohol and volatile flavor compounds. Freeze-concentrating it (as applejack was once made) would work but is probably excessive in terms of effort and reward.

The first thing I notice is that you are missing starch. Cocoa powder is 60% carbohydrates by weight. It will change the texture a lot, giving body and smoothness. The rest is fat and protein.

I would suggest that you start with 60% as much starch as your usual cocoa, and 5 tablespoons brewed coffee instead of the water. Cook your pudding - you may have to cook it through instead of using the double boiler, I don't know if cocoa starch is behaving differently, but when adding cornstarch to custard, it is normally cooked through because 1) it thickens better and 2) the yolks can "eat" your starch within a day, liquefying it, unless deactivated by heat. You can try small batches with both versions and see which one turns out better.

The fat in the cocoa is not so much, so maybe it's not worth replacing it. But if you want a fat which gives the whole thing a bit more solidity, add cocoa oil, about 15% of the original cocoa powder weight. Or even add more of it and reduce the butter for a stiffer effect.

Using your original amount of sugar and Arabica coffee should result in a recipe which isn't too sweet. But if you still have problems with sweetness, try adding small amounts of acid, maybe cream of tartar (citric acid has a noticeable flavor of its own).