Can I convert this cocoa frosting to a coffee frosting instead? Our family's go-to recipe for celebration cakes is a hazelnut torte with a semi-sweet, cocoa-based chocolate frosting. The torte is completely flourless, and pretty sweet (gotta get structure from somewhere!). Thus, the frosting is minimally sweetened to provide contrast, and to prevent the whole thing from becoming sickly-sweet.
Problem is, we have two close friends who are allergic to chocolate. A few years ago, I got the excellent suggestion to use a coffee-flavored frosting instead. We (my sister and I) tried it, and it worked as far as the flavor goes, but the frosting just wasn't as good as the chocolate original: it had texture issues, and despite doing our utmost to make sure we didn't add too much sugar, it was still too sweet.
I've looked for coffee frosting recipes, but they all seem to involve several cups of powdered sugar. Riiiight.
So, can y'all help me convert this recipe to using coffee instead of the cocoa? Bonus points if the end result is gluten-free (just to uphold the tradition, really). Also, the torte is made with 8 egg whites, so it would be nice if the converted frosting recipe would continue to use up the 8 egg yolks.

Ingredients: 2 heaping tableserving spoons Dutch-processed cocoa (the original recipe just calls for 2 big spoonfuls, but in family practice, this has morphed to a whole lot more cocoa than that), 3 to 5 tablespoons granulated sugar (depending on how heavy-handed you were with the cocoa), 5 tablespoons water, 8 egg yolks, 2 sticks unsalted butter.
Method: combine everything except the butter in the top of a double boiler. Cook slowly, stirring pretty much constantly, until very thick and sticky. (It'll take a while, and your arms will get pretty tired.) Let cool. Meanwhile, whip the butter until light and fluffy. Combine the cooled chocolate mixture and the butter.
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).