Semi-sweet chocolate, also called bittersweet, is readily available in the form of baking morsels usually used in chocolate chip cookies. Most American brands of semi-sweet chocolate have 3 1/2 tsp of sugar added to each ounce of chocolate, according to "Cookwise." Unsweetened chocolate is harder to find and it contains no added sugar. To replace unsweetened chocolate with semi-sweet in a recipe, the amount of added sugar needed to sweeten the chocolate must be taken into account.
Using a kitchen scale to verify the weight, substitute all of the unsweetened chocolate with an equal weight of semi-sweet chocolates.
Remove 1 tbsp of added sugar from the recipe for each ounce of semi-sweet chocolate used. For instance, if you replaced 227gr. of unsweetened chocolate with semi-sweet, remove 8 tbsp of sugar from the recipe.
Omit 1/3 tsp of butter or oil for each ounce of semi-sweet chocolate used. For example, remove 8/3 tsp or 2 2/3 tsp of butter from the recipe if you replaced 227gr. of unsweetened chocolate.
Finish your recipe as directed for the baking time and temperature.