Use the remaining ascending list itself as the seed:
buildUnfold :: [a] -> Tree a
buildUnfold = unfoldTree split
  where
    split [] = Nothing
    split xs =
      let (ls,x:rs) = splitAt (length xs `div` 2) xs
      in Just (ls,x,rs)
The empty seed produces Empty. Every nonempty seed produces exactly the midpoint and two child seeds used by build. Thus the defining equations agree, and structural induction on the finite input proves buildUnfold xs = build xs. The balance and ordering arguments carry over unchanged.

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