What's the best way to cover dark circles under the eyes?
I would suggest getting a corrector to apply before your concealer. These have peachy undertones so they will neutralize the dark area before you apply a concealer that is 1-2 shades lighter than your skin/foundation; without it, you may get a lil ashiness/dullness going on lol (this is from applying a too light shade to a darker area)!
PS. Bobbi Brown makes a really good one in case you want to try this out!
Hope this helps! :-D