shaheen - baking soda is used when there is an acidic ingredient in the recipe. recipes calling for orange juice, yoghurt, buttermilk, etc. sometimes call for baking soda only.
- baking powder is just baking soda with another couple of acidic ingredients. when they get wet, they react with each other and produce the gas bubbles.
chocolate is also acidic, but not extremely so, so sometimes recipes call for both. as an aside, i think that dutch process cocoa is not
acidic, or perhaps only slightly.
baking soda (sodium bicarbonate? i think that's right but don't quote me) reacts as soon as it combines with the acidic ingredient. this reaction only takes place one time, so you have to be careful not to over mix and pop all the bubbles. once the batter is mixed just enough, you need to get the cake or muffin batter into the pans or muffin cups as deftly as possible to ensure a high rise.
double-action baking powder has several ingredients, one of which also leavens a second time during the baking, so you have a little more leeway timewise.
if you're refering to that gooey cake recipe, the only chocolate in the recipe being chocolate chips, the baking soda wouldn't really be reacting to it.
as a general rule-of-thumb, i think that between 1 to 1 1/2 teaspoons of baking powder per cup of flour will give you good results. baking soda is about 3 or 4 times stronger than baking powder, so only 1/4 teaspoon or heaping 1/4 teaspoon would be about right for say, orange muffins using orange juice. that being said, often use a bit more, say a slightly heaping tablespoon of baking powder (when i'm in the measuring mood) per 2 cups of flour. you have to be careful not to go too overboard though, as it can make things a bit bitter and leave little dark spots peppered about on the surface of biscuits, cakes, etc.