Semi-Sweet Chocolate?!
Answers: I have a recipe for a cake that calls for 2 1/2 ounces of unsweetened chocolate and 1 1/2 cups of sugar. Anyone know how much sugar I would need to take away if I used semi sweet?
Substituting semi-sweet for unsweetened plus part of the sugar is definitely NOT a good plan. If you absolutely must you can substitute the semi-sweet ounce for ounce for the unsweetened (though it will change the taste dramatically and not necessarily for the better). Changing the sugar, though, completely changes the chemical composition of whatever you are baking and you might very well end up with a mess. (Baking is much touchier than just cooking.)
To make matters worse, there is no one set standard for what constitutes "semisweet" chocolate. How much sugar has been added, how much cacao solids are present and what other stuff is in there varies by brand - and it varies more than a little bit.
Add in the sugar 1/2 cup at a time. Taste the batter until you think it tastes good.
Decrease the sugar by 1 Tablespoon for each ounce of chocolate. So if you are using 2 1/2 ounces semi sweet instead of unsweetened you would need to cut back on your sugar by 2 Tablespoons plus 1 1/2 teaspoons.
Good luck!