It is always more than 50% cocoa butter. On average, it will be 55% cocoa butter and 45% cocoa solids.
No sugar is added, so the chocolate is not really edible as is. Only one ingredient should show on the package: chocolate.
Unsweetened Baking Chocolate differs from Cocoa Powder in that Cocoa Powder has most of the cocoa butter removed.
Using Unsweetened Baking Chocolate in recipes allows cooks to control the amount of sweetness that they add, and in what form (white sugar, brown sugar, honey, etc.)
Unsweetened Baking Chocolate is not the same as bittersweet chocolate. Bittersweet chocolate is chocolate has been sweetened a small amount.
You can also now buy “premelted” Unsweetened Baking Chocolate. It’s Unsweetened Baking Chocolate in a liquid form. It’s sold in a box containing several small pouches of it. It may not have, however, a high enough proportion of chocolate in it to be actually called “chocolate”: one manufacturer (Nestlé’s) labels it “Unsweetened Baking Chocolate flavour.”
Substitutes
When substituting Cocoa Powder for Unsweetened Baking Chocolate, what you have to compensate for is the fat (the cocoa butter) that was removed to make the Cocoa. The standard formula to replace 1 oz (30g) of Unsweetened Baking Chocolate with Cocoa is therefore 3 tablespoons of Cocoa Powder plus 1 tablespoon of a fat (which can be butter, oil, lard or shortening.)
To use Unsweetened Baking Chocolate when a recipe calls for Cocoa, try to find somewhere in the recipe that you can remove 1 tablespoon of fat.
Equivalents
1 envelope of “premelted” Unsweetened Baking Chocolate equals 1 square of Unsweetened Baking Chocolate.