Bindweed doesn't come up looking like butterfly-shaped leaves to me; I was thinking maybe a brassica. You might do an image search for bindweed seedlings, though, and see if they look the same, because butterflies come in various shapes too...

Uprooting bindweed is also good. Impossible to do cleanly -- the roots break off -- but the more stuff it has to regrow, above or below the ground, the sooner the roots will be exhausted.