Boil jalapeño peppers in salted water for approx. 10 minutes. Drain, cover with ice and let cool. In a food processor, mix together cream cheese, cheddar cheese and cilantro. Fill pastry bag with mixture (this really works the best). Put in refrigerator until ready to use. After peppers are cooled, slice lengthwise down one side only. Scrape out seeds and membrane. Begin to fill each pepper with mixture. DO NOT OVERFILL!
Put in refrigerator. Prepare batter.

Sift baking powder with flour, place in a large bowl. Make a well in the middle of the flour, add oil and beer, stir with a wooden spoon. Gradually add milk until incorporated. Do not overmix. Batter will become elastic. Let batter stand for 30-35 minutes, until thickened. When ready to batter peppers, in a copper bowl, whisk egg whites till stiff peaks form. Gently fold stiff whites into batter, using wooden spoon, until combined. Roll peppers in flour, then dip in batter, turning to coat thoroughly. Hold over the bowl, so excess batter can drip off. Lower peppers, one at a time into hot oil, turning once, fry until golden and crisp, 4-5 minutes. Remove to baking sheet lined with paper towel, and keep warm in the oven as you fry the rest.
Serve with sour cream and/or jalapeño jelly.