Short answer: yes, milk chocolate differs from chocolate by the milk its manufacture.
Real chocolate (as opposed to many other confections) is made from chocolate liquer, which despite the name, is not alchoholic, or even liquid at room temperature.
The fruit of the theobroma cacao tree contains seeds, known as beans. The harvested fruits are allowed to ferment, bringing many flavor and chemical changes to the beans, as well as removing the pulpy fruit. The beans are then husked for the seeds inside, the nibs.
The nibs are the first true chocolate product, although they are not sweet.
The nibs are roasted, and then ground. This is chocolate liquer, a solid colloid of cocoa fat and solid particles. It would be solid at room temperature, but the grinding process melts it.
The cocoa liquer can be pressed to separate out the cocoa butter (as for use in the cosmetics industry), leaving cocoa powder, but that is not the point of your question.
Instead, to manufacture chocolate, the liquer is conched, a mechanical process that makes the suspended particles much smaller, part of what gives chocolate its smooth mouthfeel.
Various products can be made from chocolate liquer, or cocoa butter, including:
- baking chocolate - essentially, just chocolate liquer, hardened and tempereded. May or may not be fully conched, so may not be as smooth as chocolate intended for eating.
- chocolate or dark chocolate - Chocolate liquer, possibly extra cocoa butter, and sugar. Minor optional ingredients often included are vanilla or other flavorings, and lecithen, an emulsifier.
- milk chocolate - Same as dark chocolate, with the addition of condensed milk or milk solids, depending on whether it is made via the Swiss method or the Hershey method
- white chocolate - cocoa butter, plus sugar and other flavorings
- chocolate chips - Another form of chocolate in a particular shape. Many manufacturers don't make these from true chocolate, but rather substitute another fat which doesn't melt as easily as cocoa butter, for economy, and so the chips hold their shape in the oven
- chocolate bunny - Chocolate molded into the shape of a bunny, then tempered and cooled
- German's chocolate - A brand name of quite sweet dark chocolate
Chocolate labels which list "cocoa percentage" are saying what proportion of the chocolate is cocoa liquer or additional cocoa butter or cocoa solids--that is, stuff from nibs, as opposed to sugar or other flavorings. The cocoa percentage for milk chocolate tends to be much lower than that of dark chocolates, although not every milk chocolate has a lower percentage than every dark chocolate.
See this question for information on tempering chocolate, which gives its snappy mouthfeel.
Edit: On dairy products in dark chocolate:
I was very surprised at Lemontwist's comment, so I did some googling and found this article at Go Dairy Free:
A good quality dark or semi-sweet chocolate will only have sweetener /
sugar in some form added, and may also include a touch of soy lecithin
as an emulsifier. These brands are milk-free by ingredients, but keep
in mind that most brands of chocolate are made on shared equipment.
That is, an inherently milk-free dark chocolate may be made on the
same equipment as milk chocolate. See below for my note on
cross-contamination issues.
The complications arrive as some brands of dark and semi-sweet
chocolate do include milk ingredients for a “smoother” end result.
This is particularly true in mainstream brands like Hershey’s. Some
ways that you may see milk listed in the ingredients include milk
solids, milk, milk powder, whey, butter oil or butterfat (see the Ask
Alisa post on butter oil), or even casein. If milk is in the
ingredients, it should be listed in a clearly identifiable manner per
the labeling laws, but still, use caution.
This is still in line with the information I provided, as I did mention "other flavorings" in dark chocolate, and it is not a universal or even common practice as far as I know.
For people with strong allergies, the cross-contamination issue may be more of an issue.
Vegans would have more of an issue, as lecithen is a very common ingredient in chocolate of all types, and may be animal sourced. Vegans would specifically need to reseearch and obtain chocolates that meet that standard. Googling will find many such products, but I did not find an easy single reference list.
Chocolate and cocoa powder are two different things
For drinks go with cocoa powder (Dutch process). Emulsifying the fat in chocolate is pointless and not particularly tasty. Chocolate is about 40% to 60% fat, cocoa powder is 10% to 20% fat
For the chocolate milk taste you generally want the milk fat favours, not the cocoa fat
If your cocoa powder is not bright enough, adding a little coffee will help, as will a little more roasting of the cocoa powder, but be careful, it burns quickly. I think many commercial chocolate milks use a little salt (Sodium or Ammonium) as a brightener**
In some countries you can buy "extra" or "premium dutch process" cocoa powder, it is very dark in colour, and has a lot more of that dark chocolate style flavour
** The "that tastes nice and different" reaction
Best Answer
Functionally, it should work out fine; but it is not an exact substitute.
Based on the fact that your recipe gives you a tolerance for both strength and quality of the chocolate I would say it is probably fine to proceed boldly with your plan to mix the two. You might try a small sample melt first (as suggested here) by mixing an equal but smaller amount to check your results first.
Will this create 72% (or even 70.5%) chocolate, actually no. There is more to chocolate than just the %cocoa. % of cocoa is just one element to the formula. The 72% 'on the shelf' almost certainly has different ratios of sugar, chocolate liquor, butter etc. Once melted these would react differently with the other elements of your recipe if that recipe were more exacting in it's requirements.
There is an interesting collection of articles on the chemistry of chocolate here, but nothing indicates that you should face any problems from an inexact match of 60+81 vs. 72