This dish should taste like the chocolate you use and have the texture of a mousse. I'm not familiar with Weiss chocolate, but I don't think 57% is adequate. I would suggest at least 70%.
Perhaps you should try a different brand? Again, I'm not familiar with Weiss, but is it any good? Do you like the taste of the chocolate alone? You should. If you try a different brand, I'd suggest Valrhona.
Update
The graininess is a result of over whisking. If you over whisk you can simply return it to the pot and start again.
Also, I looked up Weiss chocolate with 57% cocoa solids, and I'm more confident that this is the primary cause of your flavor problems. The breakdown for this is: 57% Cocoa, 42% Sugar, 36% Fat (the fat is included in the cocoa solid). Cocoa butter (the fat) is what carries the chocolate flavor (provided by the non-fat cocoa solids) and provides the richness and body of the chocolate. Chocolates with 57% Cocoa typically contain 33-36% fat content. Whereas chocolate with 70% cocoa solids typically have a fat content in the range of 40-42%. This lack of fat can lead to a "blander" taste in the finished dish.
Another thing to keep in mind is that this dish gives you a fair amount of wiggle room. If you whip it up and it feels like it's still lacking in some fat, then melt it, add more chocolate and start over. If you end up with something that isn't light enough, you can simply start over and add a little more water. As long as you don't burn anything you can do this indefinitely.
Finally, if you feel like experimenting you can use something besides water. In the original article Herve This recommended orange juice or blackcurrant puree.
Another Update
I found a video that uses tea in place of water. It also demonstrates how to do it without whisking, using a N₂O siphon.
Speaking from making fudge, I have gotten the most consistent results using superfine / castor sugar.
- I haven't noticed starch related issues from powdered sugar, but your mileage may vary; the problem I have had with it is due to clumping in spite of the starch (particularly in a frosting recently).
- I have used granulated sugar to make fudge and it is grainy. I melted most of it, but got sick of stirring it as it got crazy thick. I suppose it's possible to get it to the point of a completely dissolved syrup, but that would take longer than it is worth since you are likely going to run the risk of scorching.
Relative to the taste and structure, you won't get anything special from the caster sugar. However, you will get a more predictable product, and less hassle waiting for it to dissolve. You will also avoid texture problems and reduce the risks brought on by extended time in the pot (i.e. too much heat, likelier to accumulate moisture randomly).
Best Answer
1/3c Sugar is never going to dissolve in the ~one tsp liquid from the extracts, and sugar simply does not dissolve in fats - so your cocoa butter won't dissolve it either.
This is a known problem in chocolate making, both white and not... commercially sugar and/or milk powder is mixed in by a process known as conching, where the sugar is mixed into the chocolate by grinding, often for days, to the result is smooth to the tongue. Many "homemade" confections either start with premade chocolate, or add liquid to dissolve the sugar... which ends up making a fudgelike confection (you have to add other ingredients to prevent the chocolate from seizing, and end with the confection not only softer but with other flavors), some try using butter for its water content.
If the cocoa butter is just melted to or cooled to sludgy, before adding the sugar, the powdered sugar might be folded rather than mixed in and the result cool quickly enough not to settle. It still may have texture issues, though.
As for the alcohol-based aftertaste, it may be the extracts were too much for the amount of chocolate, or that your particular extracts had a heavier alcoholic taste. I believe extracts intended for candy making are different from those intended for baking, for this reason among others - they have less liquid, they are rated to higher temperatures, they balanced differently since they are less likely to be mixed into larger volumes to dilute tastes.
Many recipes start with chocolate melting discs, etc, for this reason. If you really want to make your own, or you want a flavor profile with a higher % cocoa butter and/or less sugar, you might have some success with adding cocoa butter, milk powder, and flavorings to lesser amounts of white chocolate (essentially, you'd just be using it for the amt sugar needed for the larger batch, the cocoa butter and milk powder already there would be "extras").