Before I start on the burgers, I roast poblano peppers on a cast iron skillet until the outer skin chars and separates from the pepper. I remove it and the seeds, then chop the pepper. This is a great pepper to use because it isn't very hot, but has a great and very distinct flavor.
Then I use a mister to lightly coat the buns with extra virgin olive oil. I always use buns with sesame seeds. Putting these on the grill toasts the buns as well as the seeds. The result is a great tasting, crispy toasted bun.