Substituting one chocolate for another

Moving from 55% to 70% removes about 15 percentage points of sugar and adds cocoa solids and cocoa butter in its place. In a recipe using 200g of chocolate that is roughly 30g less sugar and 30g more cacao material.

That is enough to notice. The bake will be less sweet, slightly drier because cocoa solids absorb moisture, and more intensely flavoured.

Going higher in percentage: add sugar back at roughly the difference, and consider a spoonful more fat or liquid to compensate for the drier crumb.

Going lower: reduce the recipe's sugar correspondingly, or accept a sweeter result. This direction is more forgiving because extra sweetness rarely ruins a bake.

Cocoa powder for chocolate: roughly three tablespoons of cocoa plus one tablespoon of butter or oil replaces one ounce of unsweetened chocolate. The added fat is doing necessary work.

Anything that has to set — ganache, moulded work, coating — depends on the chocolate's specific composition, and swapping types changes the ratio rather than just the flavour. See the ganache ratios rather than substituting freely.

Chips for chopped bar in a melted application never works, because chips are formulated not to flow. And compound coating and real chocolate cannot be mixed; their fats are incompatible.

In a heavily spiced, heavily sweetened or strongly flavoured bake, the specific chocolate is largely inaudible. A chilli-and-cinnamon brownie does not need a single-origin bar, and using one is money spent on something nobody will taste.

Save the good chocolate for applications where you can actually taste it — a simple ganache, a tart, or eating it.