Preheat broiler on high. Rub olive oil on the outside of the jalapeño(s) and place under the broiler on a baking sheet. Broil for about 15 minutes (turning occasionally), or until very charred and dark brown on the outside. Transfer to a bowl and cover with plastic wrap for 15 minutes. Next, remove skins from the outside and remove ribs and seeds from the inside for a milder heat; chop, and set aside.
Place eggs in a medium pot, cover with water. Bring to a boil and boil for 10 minutes. Drain water and place eggs in an ice water bath for 15 minutes. Crack and peel off the shells, cut eggs in half lengthwise, and pop the yolk out into a small mixing bowl. Set egg whites aside for now.
To the yolk bowl, add jalapeño, mayonnaise, mustard, vinegar, onion powder, salt, and pepper. Smooth with the back of a spoon and mix well.
To make a piping bag: Place egg mixture in a Ziploc bag, let all the air out, and seal. Snip off a small piece at one corner, making sure the hole will be big enough to squeeze the filling out. Pipe the yolk filling evenly among each egg. Chill well before serving.
*The additional jalapeño can be thinly sliced and placed on top of each egg for garnish. Or, finely chopped and sprinkled over eggs.