A recipe for summertime cake called for 2 cups of mixed berries. It did not specify whether the berries should be fresh or frozen. I used frozen berries. The center of the finished cake remained underdone although the outside was pretty dark. What did I do wrong?
In most recipes, you can use either fresh or frozen fruits, especially where one or the other is not recommended.
Fresh fruits may be the better pick, but sometimes frozen may offer "fresher" quality, as the fruits are picked at their prime and frozen fast at extremely low temperatures, retaining their quality.
However, you may not have followed one of the basic guidelines for cake-baking - that ingredients should be at room temperature, particularly in this case where the berries constituted a substantial proportion of the batter.
Cold berries will disrupt the flow of heat from all around the outside toward the center, which is done last. Other ingredients, such as eggs and flour (some cooks do keep it in the fridge), should also be at room temperature for the same reason.
Eggs beat up fuller at room temperature. Butter is easier to cream.
For frozen berries, you should thaw them in a colander, running a gentle stream of cold water over them if necessary, and drain them thoroughly. If there is some juice to be saved, do not run water; catch the drained juice on a plate underneath and count it towards the recipe liquid.