I am making butter chicken tonight. My recipe calls for 2 jalapenos, but the store was out and I am too cranky to go across town to another store. Can I substitute a Serrano pepper? I believe Serrano are quite a bit hotter, and I don't want to burn everyone's mouths.
Jalapenos vary widely in heat, but, yes, Serrano is usually thought of as a bit hotter. If it were me, I would use one. I would also remove the seeds and the internal membranes to cut the heat further. Maybe even start with 1/2...you can always add more.
If this is an indian recipe, the jalapenos will already be a (more or less well informed) substitution for one of the common types of green chili used in india, generally called hari mirch. Heat levels are reasonably similar comparing the two types. Generally, most types of green capsicum annuum will work (including green thai chilies. Mind the heat level, though!), but yield a subtle flavor difference. Substituting capsicum chinese types (green habanero etc...) could change the flavor profile and would need experimentation; doing away with all green chilies and only using red will leave something missing - (dried or fresh, whole or powdered) red chilies are traditionally used together with green chilies in indian recipes.
No, it's totally different. Jalapenos are much less spicy and it's also a different feel to it. Try to get jalapenos if you can. If not, try to soak the serrano peppers in vinegar for a few hours after removing the seeds and inside and see if they are a little more appropriate then. Also reduce the amount by 50% even taking that into consideration...